Digital eddy current proximity system: apparatus and method

ABSTRACT

A digital eddy current proximity system including a digital impedance measuring device for digitally measuring the proximity probes impedance correlative to displacement motion and position of a metallic target object being monitored. The system further including a cable-length calibration method, an automatic material identification and calibration method, a material insensitive method, an inductive ratio method and advanced sensing characteristics.

BACKGROUND OF THE INVENTION

[0001] Analog eddy current proximity systems which analyze and monitorrotating and reciprocating machinery are known in the art. These analogsystems typically include a proximity probe located proximate a targetobject (e.g., a rotating shaft of a machine or an outer race of arolling element bearing) being monitored, an extension cable and analogconditioning circuitry. The target, proximity probe (a noncontactingdevice which measures displacement motion and position of an observedconductive target material relative to the probe), extension cable andconditioning circuitry components are all designed to interact in such away that a voltage output from the circuitry is directly proportional toa distance between the probe and the target. This distance is commonlyreferred to as “gap”.

[0002] The interaction that takes place between these components is inaccord with the following rules: First, the electrical impedancemeasured at the conditioning circuitry is the electrical combination ofthe target, the probe including an integral sensing coil and cable, theextension cable and the conditioning circuitry. This impedance isusually called the “Tank Impedance” or parallel impedance (Zp). Second,this tank impedance is linearized and converted into a voltage directlyproportional to gap. Third, the conditioning circuitry measuresimpedance at a specific frequency that is a function of its owncircuitry. Generally, the circuitry runs at the frequency where thereactive component of the tank impedance approaches zero. In otherwords, the circuitry is a resonant system, so the frequency of operationwill be where the phase shift of the impedance is approximately zerodegrees. In reality, the phase shift is not exactly zero due to, interalia, manufacturing and component variations and tolerances of eachanalog system.

[0003] In order to compensate for these variations and tolerances, eachanalog system is required to be calibrated to have a parallel impedancewhich is as close as possible to a predefined ideal parallel impedancewhile remaining substantially unsusceptible to the multitude ofvariations and tolerances found in the target, probe, extension cable,and conditioning circuitry. Simultaneously, each analog system iscalibrated to have a maximum sensitivity to changes in gap. Moreover,each system is generally required to be calibrated to monitor onespecific target material.

[0004] These analog systems are also generally burdened by temperaturevariations in the target, the probe including the integral sensing coiland cable, the extension cable and the conditioning circuitry due to thesevere temperature variations in rotating and reciprocating machineryenvironments. Thus, each system is required to be designed around amultitude of component tolerances to compensate for the severetemperature variations engendered in these environments. Furthermore,these analog systems must also be designed around the sensitivity tochanges in the conductivity and permeability of the target, the sensingcoil, and the cable, which can greatly effect the precision of thesesystems.

[0005] Moreover, interchangeability problems arise from variations inthe target, probe, extension cable, and conditioning circuitry whichcause the tank impedance (Zp) versus gap to vary slightly from nominalresulting in a proclivity towards, inter alia, variations in incrementalscale factor (ISF), variations in average scale factor (ASF) anddeviations from a straight line (DSL). The incremental scale factor(ISF), variations in average scale factor (ASF) and deviations from astraight line (DSL) are common ways to specify transducer performance asis well known in the art.

[0006] It is critical that the displacement motion or position betweenthe target and the sensing coil of the proximity probe remains withinthe linear range of the proximity probe for providing accurate andreliable measurements over a wide range of circuit and environmentalconditions in order to operate rotating and reciprocating machinerysafely and efficiently. Heretofore, the ability to provide accurate andreliable measurements over a wide range of circuit and environmentalconditions has been dependent on, inter alia, designing andmanufacturing each production unit within close tolerances and goingthrough laborious calibration methods to compensate for the circuit andenvironmental conditions.

[0007] For the foregoing reasons, there is a need for an eddy currenttransducer system that, inter alia, substantially eliminates themanufacturing and component variations and tolerances of the prior artanalog systems, a system that provides correct gap reading for differenttarget materials and a system which is easy to calibrate.

[0008] Additionally, there is a need to solve the general problem ofcompensating for temperature errors, temperature profiles of differenttarget materials and changes in component conductivity and permeabilityin order to preclude anomalous behavior in eddy current transducersystems.

[0009] Furthermore, there is a need for an eddy current transducersystem that has better linearity and interchangeability. Moreover, thereis a need for an eddy current transducer system that does not requirecomponent changes when re-calibrated to a new or different targetmaterial.

SUMMARY OF THE INVENTION

[0010] The instant invention is distinguished over the known prior artin a multiplicity of ways. For one thing, the instant invention providesa unique digital system for digitally measuring an unknown electricalimpedance. Additionally, the instant invention provides a digitalproximity system that is a direct one for one replacement for existinganalog eddy current proximity systems which is compatible with anyexisting (or future) eddy current proximity probe (a noncontactingdevice which measures displacement motion and position of an observedconductive or metallic target material relative to the probe) andextension cable assembly. Thus, the instant invention can directlyreplace the analog conditioning circuitry of prior art analog systemsthereby eliminating the anomalies associated with manufacturing andcomponent variations, and tolerances of these systems. Furthermore, theinstant invention eliminates the laborious design and calibrationmethods required to calibrate prior art analog systems in order tocompensate for manufacturing and component variations, tolerances andenvironmental conditions.

[0011] In one form, the instant invention provides a system whichincludes a unique voltage ratio apparatus and method for digitallymeasuring an unknown electrical component value. The system accomplishesthis by digitizing a first voltage impressed across a serial coupling ofa first electrical component and a second electrical component, and bydigitizing a second voltage impressed across the second electricalcomponent only. Each of the two digitized voltages is then convolvedwith digitized waveforms to obtain a first and a second complex voltagenumber. A ratio of the second complex number to a difference between thefirst and the second complex number is determined and multiplied by aknown value of the first electrical component to determine the unknownvalue of the second electrical component. A resistance means having aknown value can be employed as the first electrical component. Thesecond electrical component can take the form of a proximity probehaving an unknown impedance value which, when determined by the instantinvention, can be correlated to a distance between the probe and ametallic target object being monitored by the probe. Iterativelyrepeating the voltage ratio method results in continuously digitallydetermining the unknown impedance values of the probe which can bedirectly correlated to the continuous displacement motion and positionof the target being monitored relative to the probe. In one form, thedigitally determined impedance values can be transformed into analogsignals and used to trip alarms, circuit breakers, etc., when thesignals are outside nominal operating ranges set by plant operators.

[0012] Additionally, the instant invention provides a system that can beused as a direct one for one replacement for existing (and future)analog eddy current proximity systems. The system includes a uniqueapparatus and method for digitally measuring the impedance of aproximity probe and an extension cable (if employed) which includes theunique voltage ratio apparatus and method delineated supra to obtain anunknown impedance of the proximity probe. Then, the system mirrors acircuit equivalent impedance of an existing (or future) analog proximitycircuit and combines the measured impedance with the circuit equivalentimpedance for defining a parallel or tank impedance. The defined tankimpedance is then correlated to a distance between the probe and ametallic target object being monitored by the probe. Hence, the systemcan continuously digitally determine the unknown impedance value of theprobe by iteratively repeating the aforementioned method and thencorrelate the digitally measured probe impedance values to thecontinuous displacement motion and position of the target for analyzingand monitoring rotating and reciprocating machinery.

[0013] More particularly, the instant invention provides a system whichemploys at least one eddy current proximity probe having a multi-axialprobe cable coupled to a sensing coil located proximate a conductivetarget to be monitored. The sensing coil is coupled to ground and to asecond terminal of a resistor via the probe cable and an extension cable(if employed). A first terminal of the resistor is coupled to a signalgenerator device that is digitally programmable to generate dynamicdriving signals.

[0014] The signal generator device can be included in a digital feedbackloop which includes means for monitoring the phase of the tank impedanceand to provide corrective action (a frequency change) for adjusting thatphase. Thus, the signal generator device can be digitally programmed toemulate the operating frequency of any previous (or future) analogproximity system and can also be digitally reprogrammed, in real time,for driving the sensing coil of the probe at one or more frequenciescorrective of any anomalous phase shift calculated from the probe ortank impedance or due to any other anomalies within the system. Forexample, the instant invention can drive the sensing coil at a precisefrequency corrective of temperature variations in the probe includingthe integral sensing coil and probe cable, and in the target.

[0015] A filter is interposed between the signal generator device andthe first terminal of the resistor to purify the output dynamic signalsof the signal generator device by eliminating, inter alia, harmonicsthat are created in the device. In addition, the filter helps reduce thenoise bandwidth of the system which improves a signal to noise ratio.The filtered signal is driven through the resistor, extension cable (ifemployed), probe cable and coil for inducing eddy currents within thetarget. In turn, the eddy currents in the target induce a voltage in thesensing coil of the probe and hence, a change in an impedance of theprobe and extension cable (if employed) which varies as a function of,inter alia, the displacement motion and position of the target relativeto the probe.

[0016] The first and second terminals of the resistor are coupled toinputs of a first and a second analog to digital converter respectively.In turn, the outputs of the analog to digital converters are coupled toa digital signal processor including a convolution means. The firstanalog to digital converter receives and samples a first voltage betweenthe serially coupled resistor, extension cable (if employed), probecable and coil and outputs a first digital voltage signal to the digitalsignal processor. The second analog to digital converter receives andsamples the voltage between ground and the combination of the extensioncable (if employed), the probe cable and the coil and then, outputs asecond digitized voltage signal to the digital signal processor. Atiming control means is operatively coupled to the analog to digitalconverters and to the signal generator device such that the sampling issynchronously performed with the driving signal of the signal generator.This ensures, inter alia, that when the voltages are calculated therewill be exactly one cycle worth of data stored in each data set.

[0017] The digital signal processor convolves the two digitized voltagesby convolving each digitized voltage with a digital sine and cosine waveto obtain a first and a second complex voltage number. Once theconvolution of the digitized voltages is performed the impedance valueof the extension cable (if employed), probe cable and coil can becalculated directly from the measured voltages.

[0018] The system includes an open/short/load calibration method whichcan compensate for cable length included in the second electricalcomponent. For example, the extension cable can be compensated for byusing the open/short/load calibration method according to the instantinvention. Thus, the system can apply the open/short/load calibrationmethod to the measured impedance to obtain a compensated impedance.Furthermore, the open/short/load calibration method can be utilized tocalibrate each printed wire assembly within the system.

[0019] The measured impedance or the compensated impedance is thencorrelated by the system to a gap value by using equations, numericalmethods, algorithmic functions or lookup tables wherein gap values arecorrelated to measured or compensated impedance values defining the gapor spacing interposed between the probe and the target being monitored.This method of measuring gap can be continuously repeated formonitoring, for example the vibration of a rotating shaft of a machineor an outer race of a rolling element bearing.

[0020] Additionally, the system can combine the measured impedance orthe compensated impedance value with a mathematical model value or anempirically predetermined value of an existing (or future) analogconditioning circuit that is compatible with the particular probe beingemployed. This value can be called up from a memory means associatedwith the digital signal processor. The digital signal processor combinesthis value with the measured impedance or the compensated impedance toobtain a resultant impedance defined as the tank impedance. This tankimpedance can be employed to determine the gap between the probe and thetarget by using equations, numerical methods, algorithmic functions orlookup tables wherein gap values are correlated to tank impedancevalues. Thus, the existing proximity probe can be retained and thismethod of measuring gap can be continuously repeated for monitoring, forexample the vibration of a rotating shaft of a machine or an outer raceof a rolling element bearing that was heretofore monitored by an analogeddy current proximity system.

[0021] Gap values can be outputted to a digital to analog converter forproviding analog outputs or downloaded to a processing stage for furtherprocessing and/or providing digital and/or analog outputs.

[0022] The impedance value of analog conditioning circuitry determinedfrom the mathematical model or empirically is typically dependent onoperating frequency. Thus, once the tank impedance is determined it canbe used to determine if the system is running at the proper frequency.If the system is not running at the proper frequency the digitalfeedback loop can be used to feedback a signal from the digital signalprocessor to program the signal generator device for dynamicallyadjusting the driving signal.

[0023] Moreover, the instant invention includes a unique materialidentification method for automatically identifying a target materialand automatically calibrating itself to monitor the identified materialthereby eliminating the need for component changes and laboriousre-calibration methods inherent with prior art systems. The instantinvention also expands the unique material identification method toinclude a material insensitive method which is capable of outputting agap value substantially correct for any target material being monitoredthereby providing a material insensitive digital proximity system. Thus,the instant invention provides a digital proximity system that does notrequire component changes when being used to replace an existing systemand/or does not require re-calibration when being used with a new ordifferent target material. As a result, the instant invention provides adigital proximity system which can not be mis-calibrated when put intooperation and which eliminates the interchangeability problem found inprior art systems.

[0024] Additionally, the instant invention includes a unique inductiveratio method which allows a gap versus inductive ratio curve to bedetermined for a specific target material without knowing the far gapimpedance of the probe coil and thus, without removing the probe from amachine being monitored. The gap versus inductive ratio curve determinedby this method can be used to determine the gap between the probe andthe target being monitored. Furthermore, this method can be used todiscern moisture ingress within a probe while it is still in themachine.

OBJECTS OF THE INVENTION

[0025] Accordingly, a primary object of the instant invention is toprovide a new, novel and useful digital eddy current proximity system:apparatus and method.

[0026] Another further object of the instant invention is to provide isto provide a new, novel and useful digital system for measuring anunknown electrical value of an electrical component, for example, anunknown impedance value of an electrical component thereby providing adigital impedance measuring device.

[0027] Another further object of the instant invention is to provide adigital proximity system as characterized above which includes thedigital impedance measuring device employed to measure impedance of aneddy current displacement probe and correlate the measured impedance toa gap between the probe and a target being monitored.

[0028] Another further object of the instant invention is to provide adigital proximity system as characterized above which includes means todynamically measure the impedance of an eddy current probe at bandwidthshigh enough to support vibration information.

[0029] Another further object of the instant invention is to provide adigital proximity system as characterized above which provides a digitalproximity system that is compatible with previous (or future) analogeddy current systems and existing signal conditioning sensors includingproximity sensors using one or more frequencies.

[0030] Another further object of the instant invention is to provide adigital proximity system as characterized above which is capable ofemulating the operation of analog conditioning circuitry of eddy currentproximity systems for providing backwards (or future) compatibility withanalog systems.

[0031] Another further object of the instant invention is to provide adigital proximity system as characterized above which includes anopen/short/load calibration method which allow various cable lengthcompatibility.

[0032] Another further object of the instant invention is to provide adigital proximity system as characterized above which includes a uniqueautomatic material identification and calibration method.

[0033] Another further object of the instant invention is to provide adigital proximity system as characterized above which includes a uniquematerial insensitive method.

[0034] Another further object of the instant invention is to provide adigital proximity system as characterized above which includes a uniqueinductive ratio method for measuring gap values.

[0035] Another further object of the instant invention is to provide adigital proximity system as characterized above which is self-contained,self-configuring and self-analyzing.

[0036] Another further object of the instant invention is to provide adigital proximity system as characterized above which is capable ofidentifying an eddy current displacement probe that is coupled thereto.

[0037] Viewed from a first vantage point, it is an object of the instantinvention to provide a device for digitally measuring electricalimpedance, comprising in combination: a network including a firstelectrical component and a second electrical component seriallyconnected; a signal generating means operatively coupled to the networkfor driving a current through the serially connected components; meansfor sampling a first voltage impressed across the network and a secondvoltage impressed across the second component into digitized voltages;means for convolving each the digitized voltage with a digital waveformfor forming a first complex number and a second complex numbercorrelative to the first voltage impressed across the network and thesecond voltage impressed across the second component respectively; meansfor determining a ratio of the second complex number to a differencebetween the first and the second complex number, and means forcalculating an electrical impedance of the second component bymultiplying the ratio by a value of the first component wherein theelectrical impedance of the second component is digitally measured.

[0038] Viewed from a second vantage point, it is an object of theinstant invention to provide a method for digitally measuring electricalimpedance, the steps including: forming a network including providing afirst electrical component and a second electrical component seriallyconnected; driving the network with a dynamic signal for impressing avoltage across the network and each component; digitizing the voltageacross the network and the voltage across the second electricalcomponent; convolving each of the digitized voltages with a digitalwaveform for forming a first complex number and a second complex numbercorrelative to the voltages across the network and across the secondelectrical component respectively; determining a ratio of the secondcomplex number to a difference between the first complex number and thesecond complex number; calculating an electrical impedance of the secondelectrical component by multiplying the ratio by a know digitized valueof the first electrical component wherein the electrical impedance ofthe second component is digitally measured.

[0039] Viewed from a third vantage point, it is an object of the instantinvention to provide an apparatus for determining a gap between aproximity probe and a conductive target material, the apparatuscomprising in combination: a network including a first electricalcomponent and a proximity probe serially connected; a signal generatingmeans operatively coupled to the network for driving a current throughthe serial connection wherein a first analog voltage is impressed acrossthe network and a second analog voltage is impressed across theproximity probe; means for sampling and digitizing the first analogvoltage impressed across the network and the second analog voltageimpressed across the proximity probe into digitized voltages; means forconvolving each the digitized voltage with a digital waveform forforming a first complex number and a second complex number correlativeto the first analog voltage impressed across the network and the secondanalog voltage impressed across the proximity probe respectively; meansfor determining a voltage ratio of the second complex number to adifference between the first complex number and the second complexnumber; means for processing the voltage ratio into a gap valuecorrelative to a gap between the proximity probe and a conductive targetmaterial.

[0040] Viewed from a fourth vantage point, it is an object of theinstant invention to provide an apparatus for determining a gap betweena proximity probe and a conductive target material, the apparatuscomprising in combination: a network including an extension cableinterposed between and serially connected to a first electricalcomponent and a proximity probe; a signal generating means operativelycoupled to the network for driving a current through the serialconnection wherein a first analog voltage is impressed across thenetwork and a second analog voltage is impressed across the serialconnection of the extension cable and the proximity probe; means forsampling and digitizing the first analog voltage impressed across thenetwork and the second analog voltage impressed across the serialconnection of the extension cable and the proximity probe into digitizedvoltages; means for convolving each the digitized voltage with a digitalwaveform for forming a first complex number and a second complex numbercorrelative to the first analog voltage impressed across the network andthe second analog voltage impressed across the serial connection of theextension cable and the proximity probe respectively; means fordetermining a voltage ratio of the second complex number to a differencebetween the first complex number and the second complex number; meansfor processing the voltage ratio into a gap value correlative to a gapbetween the proximity probe and a conductive target material.

[0041] Viewed from a fifth vantage point, it is an object of the instantinvention to provide an apparatus for determining a dynamic gaps betweena proximity probe and a conductive target material, the apparatuscomprising in combination: means for establishing dynamic voltagesignals correlative to dynamic gaps between a proximity probe and aconductive target material; sampling means for digitizing theestablished dynamic voltage signals into digital voltage signals; adigital multiplier for multiplying each the digital voltage signal by adigital sine signal and a digital cosine signal; means for accumulatingvalues of each multiply in a memory, and means for processing eachmultiply for obtaining complex voltage representations correlative todynamic gaps between the proximity probe and a conductive targetmaterial.

[0042] Viewed from a sixth vantage point, it is an object of the instantinvention to provide a method for measuring a gap between a proximityprobe and a conductive target material, the method including the stepsof: providing a network of components including a first electricalcomponent and a proximity probe component serially connected; driving adynamic current through the serially connected electrical components forimpressing a first analog voltage across the network and a second analogvoltage cross the proximity probe component; sampling and digitizing thefirst analog voltage impressed across the serially connected resistanceand probe components to obtain a first digitized voltage value; samplingand digitizing a second analog voltage impressed across the probecomponent to obtain a second digitized voltage value; digitallyconvolving the first digitized voltage and the second digitized voltageinto a first complex number and a second complex number respectively;calculating a voltage ratio of the second complex number to a differencebetween the first complex number and the second complex number;processing the voltage ratio into a gap value correlative to a gapbetween the proximity probe and a conductive target material.

[0043] Viewed from a seventh vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the method includingthe steps of: providing a network of components including a firstelectrical component and a proximity probe component serially connected;driving a dynamic current through the serially connected electricalcomponents including the resistance component and the proximity probecomponent for impressing a first analog voltage across the network and asecond analog voltage cross the proximity probe component; sampling anddigitizing the first analog voltage impressed across the seriallyconnected resistance and probe components to obtain a first digitizedvoltage value; sampling and digitizing a second voltage impressed acrossthe probe component to obtain a second digitized voltage value;digitally convolving the first digitized voltage and the seconddigitized voltage into a first complex number and a second complexnumber respectively; calculating a voltage ratio of the second complexnumber to a difference between the first complex number and the secondcomplex number; multiplying the voltage ratio by a value of the firstelectrical component for determining an impedance of the proximityprobe; correlating the determined impedance of the proximity probe to agap between the proximity probe and a conductive target material.

[0044] Viewed from a eighth vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the method includingthe steps of: providing a network of components including a firstelectrical component, an extension cable component and a proximity probecomponent respectively serially connected, and locating the proximityprobe adjacent a conductive target material; driving a dynamic currentthrough the serially connected electrical components for impressing afirst analog voltage across the network and a second analog voltageacross the serial connection of the extension cable component and theproximity probe component; sampling and digitizing the first analogvoltage impressed across the network to obtain a first digitized voltagevalue; sampling and digitizing a second analog voltage impressed acrossthe serial connection of the extension cable component and the proximityprobe component to obtain a second digitized voltage value; digitallyconvolving the first digitized voltage value and the second digitizedvoltage value into a first complex number and a second complex numberrespectively; calculating a voltage ratio of the second complex numberto a difference between the first complex number and the second complexnumber; processing the voltage ratio into a gap value correlative to agap between the proximity probe and the conductive target material.

[0045] Viewed from a ninth vantage point, it is an object of the instantinvention to provide a method for measuring a position of a conductivetarget material, the steps including: sampling and digitizing a firstvoltage impressed across a serial connection of a resistance means and aproximity probe located adjacent a conductive target material to obtaina first digitized voltage; sampling and digitizing a second voltageimpressed only across the probe to obtain a second digitized voltage,transforming the two digitized voltages into complex voltage numbers;calculating an electrical impedance of the proximity probe by using bothcomplex voltage numbers; correlating the calculated electrical impedanceto a gap between the proximity probe and the conductive target material.

[0046] Viewed from a tenth vantage point, it is an object of the instantinvention to provide a method for measuring a gap between a proximityprobe and a conductive target material, the steps including: samplingand digitizing a first voltage impressed across a serial connection of afirst electrical component and a proximity probe located adjacent aconductive target material to obtain a first digitized voltage; samplingand digitizing a second voltage impressed across the probe to obtain asecond digitized voltage, transforming the two digitized voltages intocomplex voltage numbers; determining an electrical impedance of theproximity probe by using both complex voltage numbers; normalizing theelectrical impedance of the proximity probe; correlating the normalizedelectrical impedance of the proximity probe to a gap between theproximity probe and the conductive target material.

[0047] Viewed from a eleventh vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the steps including:sampling and digitizing a first voltage impressed across a serialconnection of a first electrical component, an extension cable and aproximity probe located adjacent a conductive target material to obtaina first digitized voltage; sampling and digitizing a second voltageimpressed across the probe to obtain a second digitized voltage,transforming the two digitized voltages into complex voltage numbers;determining an electrical impedance of the proximity probe by using bothcomplex voltage numbers and compensating for the extension cable;normalizing the electrical impedance of the proximity probe; correlatingthe normalized electrical impedance of the proximity probe to a gapbetween the proximity probe and the conductive target material.

[0048] Viewed from a twelfth vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the method includingthe steps of: digitally measuring an electrical impedance of a proximityprobe located adjacent a conductive target material; combining apredetermined digitized impedance with the digitally measured impedanceof the proximity probe; correlating the combined impedance to a gapinterposed between the proximity probe and the conductive targetmaterial being monitored.

[0049] Viewed from a thirteenth vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the method includingthe steps of: digitally measuring an electrical impedance of an aproximity probe and an extension cable connected thereto, the proximityprobe is located adjacent a conductive target material; combining apredetermined digitized impedance with the digitally measured impedance;correlating the combined impedance to a gap interposed between theproximity probe and the conductive target material being monitored.

[0050] Viewed from a fourteenth vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the method includingthe steps of: measuring an impedance of a proximity probe locatedproximate a conductive target material and an extension cableoperatively coupled to the proximity probe; compensating the measuredimpedance by using compensation coefficients stored in a memory means;combining a predetermined impedance with the compensated measuredimpedance for forming a combination impedance; determining a gap betweenthe proximity probe and the conductive target material as a function ofthe combination impedance; iteratively repeating the measuring,compensating, combining and determining steps to substantiallycontinuously monitor the gap between the probe and the target as afunction of the combination impedance.

[0051] Viewed from a fifteenth vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the steps including:providing a database of normalized impedance curve representations fordifferent conductive target materials; measuring an impedance of aproximity probe located proximate a conductive target material beingidentified; normalizing the measured probe impedance; utilizing thenormalized probe impedance and the database of normalized impedancecurve representations for identifying the conductive target material;determining a gap value between the proximity probe and the conductivetarget material from the normalized probe impedance and the identifiedtarget material.

[0052] Viewed from a sixteenth vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the steps including:providing a representation of a defined series of gap locus eachrepresentative of the same gap for different target materials; measuringan impedance of a proximity probe located proximate a conductive targetmaterial; normalizing the measured probe impedance; determining a gapvalue between the proximity probe and the conductive target materialfrom the normalized probe impedance and the representation of thedefined series of gap locus wherein the gap value is substantiallycorrect for any conductive target material adjacent the proximity probethereby providing a material insensitive method for measuring gap valuesbetween the proximity probe and different conductive target materials.

[0053] Viewed from a seventeenth vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the steps including:providing a representation of a defined series of gap locus eachrepresentative of the same gap for different target materials; measuringan impedance of a proximity probe located proximate a conductive targetmaterial, the proximity probe including a probe cable; compensating animpedance contribution of the probe cable from the measured probeimpedance to define a measured coil impedance; normalizing the measuredcoil impedance; determining a gap value between the proximity probe andthe conductive target material from the normalized coil impedance andthe representation of the defined series of gap locus wherein the gapvalue is substantially correct for any conductive target materialadjacent the proximity probe thereby providing a material insensitivemethod for measuring gap values between the proximity probe anddifferent conductive target materials.

[0054] Viewed from a eighteenth vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the steps including:measuring a proximity probe impedance at a first frequency and a seconddifferent frequency, the proximity probe including an integral sensingcoil; determining an impedance of the sensing coil from the measuredproximity probe impedance at the first frequency and the seconddifferent frequency; dividing a reactance of the impedance of thesensing coil at the first frequency by the reactance of the impedance ofthe sensing coil at the second different frequency for defining aninductive ratio; correlating the inductive ratio to a valuerepresentative to a gap between the proximity probe and the conductivetarget material.

[0055] Viewed from a nineteenth vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the steps including:sampling and digitizing a first voltage impressed across a serialconnection of a resistance means and a proximity probe located adjacenta conductive target material to obtain a first digital voltagecorrelative to the first voltage at a first frequency; sampling anddigitizing a second voltage impressed only across the probe to obtain asecond digital voltage correlative to the second voltage at the firstfrequency, digitally convolving the first digital voltage and the seconddigital voltage into a first complex voltage number and a second complexvoltage number; calculating an electrical impedance of the proximityprobe at the first frequency by using the first complex voltage numberand the second complex voltage number; sampling and digitizing a thirdvoltage impressed across the serial connection of the resistance meansand the proximity probe located adjacent the conductive target materialto obtain a third digital voltage correlative to the third voltage at asecond frequency; sampling and digitizing a fourth voltage impressedonly across the probe to obtain a fourth digital voltage correlative tothe fourth voltage at the second frequency, digitally convolving thethird digital voltage and the fourth digital voltage into a thirdcomplex voltage number and a fourth complex voltage number; calculatinga complex electrical impedance of the proximity probe at the secondfrequency by using the third complex voltage number and the fourthcomplex voltage number; dividing a reactance of the calculated complexelectrical impedance of the sensing coil at the first frequency by thereactance of the calculated complex electrical impedance of the sensingcoil at the second different frequency for defining an inductive ratio;correlating the inductive ratio to a value representative to a gapbetween the proximity probe and the conductive target material.

[0056] Viewed from a twentieth vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the steps including:providing a proximity probe having a first end located adjacent aconductive target material and having a second end coupled to a firstend of an extension cable; measuring an impedance at a second end of theextension cable; compensating the measured impedance by mathematicallyeliminating extension cable residuals from the measured impedance fordefining a proximity probe impedance of the proximity probe; correlatingthe proximity probe impedance with a value representative of a gapbetween the proximity probe and the conductive target material.

[0057] Viewed from a twenty-first vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the steps including:providing an extension cable having two ends; determining a firstimpedance of the extension cable with one of the two ends opened fordefining an open impedance; determining a second impedance of theextension cable with one of the two ends shorted for defining a shortimpedance; providing a proximity probe having an end located adjacent aconductive target material and having an opposite end coupled to one ofthe two ends of the extension cable; measuring an impedance at the otherend of the extension cable; determining an impedance of the proximityprobe as a function of the short impedance, the open impedance and themeasured impedance for defining a proximity probe impedance; correlatingthe proximity probe impedance with a value representative of a gapbetween the proximity probe and the conductive target material.

[0058] Viewed from a twenty-second vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the steps including:providing an extension cable having two ends; determining a firstimpedance of the extension cable with one of the two ends opened fordefining a open impedance; determining a second impedance of theextension cable with one of the two ends shorted for defining a shortimpedance; determining a third impedance of the extension cable with oneof the two ends coupled to a load having a known value for defining aload impedance; providing a proximity probe having an end locatedadjacent a conductive target material and having an opposite end coupledto one of the two ends of the extension cable; measuring an impedance atthe other end of the extension cable; determining an impedance of theproximity probe as a function of the short impedance, the openimpedance, the load impedance and the measured impedance for definingthe proximity probe impedance; correlating the proximity probe impedancewith a value representative of a gap between the proximity probe and theconductive target material.

[0059] Viewed from a twenty-third vantage point, it is an object of theinstant invention to provide a method for measuring a gap between aproximity probe and a conductive target material, the steps including:providing an extension cable having two ends; determining a first loadimpedance of the extension cable with one of the two ends coupled to afirst load; determining a second load impedance of the extension cablewith one of the two ends coupled to a second load; the second loadhaving an impedance that is less than the impedance of the first load;providing a proximity probe having an end located adjacent a conductivetarget material and having an opposite end coupled to one of the twoends of the extension cable; measuring an impedance at the other end ofthe extension cable; calculating a proximity probe impedance of theproximity probe as a function of the measured impedance, the first loadimpedance and the second load impedance for compensating for extensioncable residuals; correlating the proximity probe impedance with a valuerepresentative of a gap between the proximity probe and the conductivetarget material.

[0060] Viewed from a twenty-fourth vantage point, it is an object of theinstant invention to provide a method for measuring a characteristic ofa conductive target material disposed adjacent a proximity probe, thesteps including: providing a length of cable having a first end and asecond end; determining a first impedance of the cable with the firstend opened for defining a open impedance; determining a second impedanceof the cable with the first end shorted for defining a short impedance;coupling the first end of the cable to a proximity probe and having thesecond end of the cable coupled to a digital eddy current proximitysystem; measuring, at the second end of the cable, an impedance of thecoupled cable and proximity probe; calculating the proximity probeimpedance as a function of the measured impedance, the open impedance,and the short impedance for compensating for cable length residuals;correlating the proximity probe impedance with a characteristic of aconductive target material disposed adjacent the proximity probe.

[0061] These and other objects and advantages will be made manifest whenconsidering the following detailed specification when taken inconjunction with the appended drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0062]FIG. 1 is a block diagram of the digital proximity systemaccording to the instant invention shown employing a proximity probeincluding a sensing coil located proximate a conductive target object tobe monitored.

[0063]FIG. 2 is a block diagram of the digital proximity systemaccording to the instant invention providing further detail.

[0064]FIG. 3 is a block diagram of the digital proximity systemaccording to the instant invention shown employing an extension cable.

[0065]FIG. 4 is a flow chart of a voltage ratio method according to theinstant invention.

[0066]FIG. 5 is a diagrammatical view of a convolution block of thedigital proximity system according to the instant invention.

[0067]FIG. 6 is a schematic of an example of analog conditioningcircuitry of an analog eddy current proximity system.

[0068]FIG. 7 is a schematic showing an equivalent circuit of the analogconditioning circuitry shown in FIG. 6.

[0069]FIG. 8 is a partial schematic of that which is shown in FIG. 7 forshowing where a feedback voltage is applied within the analogconditioning circuitry.

[0070]FIG. 9 is a general flow chart of a resonant method according tothe instant invention.

[0071]FIG. 10 is a flow chart of a cable compensation and gapmeasurement method according to the instant invention.

[0072]FIG. 11 is a block diagram of an open/short/load compensationmodel including a four-terminal circuit block according to the instantinvention.

[0073]FIG. 12 is a diagram of a cable(s) replacing the four-terminalcircuit block shown in FIG. 11 and with the cable(s) in an opencondition according to the instant invention.

[0074]FIG. 13 is a diagram of the cable(s) replacing the four-terminalcircuit block shown in FIG. 11 and with the cable(s) in a shortcondition according to the instant invention.

[0075]FIG. 14 is a diagram of the cable(s) replacing the four-terminalcircuit block shown in FIG. 11 and with the cable(s) coupled to a loadaccording to the instant invention.

[0076]FIG. 15 is an exemplary graph showing a normalized impedancediagram.

[0077]FIG. 16 is a flow chart of the material identification andcalibration method according to the instant invention.

[0078]FIG. 17 is an exemplary graph showing normalized impedances ofdifferent materials, which is employed in the material identificationand calibration method according to the instant invention.

[0079]FIG. 18 is an exemplary graph showing normalized impedances ofdifferent materials and showing a series of gap locus employed by thematerial insensitive method according to the instant invention.

[0080]FIG. 19 is a flow chart of the material insensitive methodaccording to the instant invention.

[0081]FIG. 20 is a graph showing a normalized impedance plane ofresistance and reactance for diagrammatically defining nomenclature ofan inductive ratio method according to the instant invention.

[0082]FIG. 21 is a graph showing an inductive ratio as a function of gapfor defining nomenclature of the inductive ratio method according to theinstant invention.

[0083]FIG. 22 is a flow chart of an inductive ratio method according tothe instant invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0084] Considering the drawings, wherein like reference numerals denotelike parts throughout the various drawing figures, reference numeral 10is directed to the digital proximity system according to the instantinvention.

[0085] In its essence, and referring to FIGS. 1 through 4, the system 10includes, inter alia, a unique voltage ratio apparatus and method (VRmethod) for digitally measuring an unknown electrical value of anelectrical component. In one preferred form, the system 10 samples anddigitizes a dynamic voltage V₁ impressed across a serial coupling of afirst electrical component having a known electrical value and a secondelectrical component having an unknown electrical value. Additionally,the system 10 samples and digitizes a dynamic voltage V₂ impressed onlyacross the second electrical component. These two digital voltages arethen digitally convolved by the system 10 into complex voltage numbersV_(1C) and V_(2C) respectively. A ratio of the second complex number toa difference between the first and the second complex number is thendetermined by the system 10. The system 10 can use this voltage ratio todetermine the unknown electrical value of the second electricalcomponent. For example, and according to the instant invention, thesystem 10 can employ a proximity probe as the second electricalcomponent and continuously digitally measure an impedance value of theprobe monitoring, for example, a rotating shaft of a machine or an outerrace of a rolling element bearing. The digitally measured impedancevalues can then be correlated by the system 10 to displacement motionand position of the rotating shaft or the outer race of the rollingelement bearing relative to the probe for monitoring machinery status.

[0086] Specifically, and referring to FIGS. 1 through 4, the system 10employs a proximity probe 12 which is disposed proximate a conductive ormetallic target object T (e.g., a rotating shaft of a machine or anouter race of a rolling element bearing) to be monitored. The system 10samples and digitizes a voltage V₁, impressed across a serial couplingof a resistance means 40 having a known electrical resistance value Rand the proximity probe 12 (and, when employed, an extension cable 30coupled to the probe 12) having an unknown electrical value Z_(unknown)into a digital voltage V_(1D). Additionally, the system 10 samples anddigitizes a voltage V₂ impressed only across the proximity probe 12 (andcoupled extension cable when employed) into a digital voltage V_(2D).The system 10 then digitally convolves the two digital voltages V_(1D),V_(2D) into first and second complex voltage numbers V_(1C) and V_(2C)respectively. Then, the system 10 determines a ratio of the firstcomplex number to a difference between the first and the second complexnumber and multiplies this voltage ratio by the known electricalresistance value R to determine the unknown impedance value Z_(unknown)of the probe 12. This process follows the equationZ_(unknown)=[V_(2C)/(V_(1C)−V_(2C))]*R. The determined impedanceZ_(unknown) can then be compensated by using an open/short oropen/short/load calibration or compensation method according to theinstant invention which will be described in detail infra. The system 10then correlates the determined impedance or the compensated determinedimpedance to a gap interposed between the probe disposed proximate themetallic target object T being monitored. Iteratively repeating thevoltage ratio method results in continuously determining the unknownimpedance values of the probe which can be correlated into valuesrepresentative of the displacement motion and position of the metallictarget object T relative to the probe. Thus, the system 10 according tothe instant invention can be employed as, inter alia, a digitalproximity system for continuously monitoring rotating and reciprocatingmachinery.

[0087] More particularly, and referring to FIGS. 1 through 4, thedigital proximity system 10 includes the unique voltage ratio method (VRmethod) for digitally measuring the unknown electrical impedance of theprobe 12 operatively coupled to the system 10 and strategically coupledto a machine for sensing raw dynamic data that is correlative to thespacing between the probe and the conductive or metallic target object T(e.g., a rotating shaft of a machine or an outer race of a rollingelement bearing) being monitored. The digital proximity system 10includes the resistance means 40 having a value R, a filter means 50, abuffer, gain and offset means 60 and a signal generator means 70. Thedigital proximity system 10 further includes a timing control means 80,a sampling means 90, a convolution means 100 and a digital signalprocessor (DSP) means 110.

[0088] The resistance means 40 includes a first terminal 41 and a secondterminal 42 respectively coupled between a first node 44 and a secondnode 46. The proximity probe 12 includes an unknown dynamic probeimpedance having a value Z_(unknown) and is coupled between the secondterminal 42 of the resistance means 40 at node 46 and a ground node 48.Thus, the resistance means 40 and the probe 12 form a serial connection.

[0089] The probe 12 includes an integral sensing element or coil 14 anda multi-conductor probe cable 20. The sensing element 14 includes afirst electrical lead 16 and a second electrical lead 18. The probecable 20 includes a first conductor 22 and a second conductor 24extending from a first end 26 to a second end 28 of the probe cable 20.The first conductor 22 and the second conductor 24 at the first end 26of the cable 20 are operatively coupled to the first electrical lead 16and the second electrical lead 18 of the sensing element 14respectively. The first conductor 22 at the second end 28 of the cable20 is coupled to the second terminal 42 of the resistance means 40 atnode 46 and the second conductor 24 is coupled to the ground node 48thereby grounding one lead of the unknown dynamic probe impedanceZ_(unknown). It is important to note that the configuration of theresistance means and the unknown probe impedance Z_(unknown) is neitherarbitrary nor inconsequential. The configuration of the instantinvention grounds a conductor of the probe so as to not have bothconductors varying thereby eliminating, inter alia, signal changeswithin the probe due to, for example, external influences such as onemoving or grabbing the cable. Furthermore, the unknown probe impedanceZ_(unknown) is digitally measured by direct read circuitry which takesdirect voltage readings rather then inferring the voltage across or thecurrent through the probe.

[0090] Additionally, the instant invention can employ a multi-conductorextension cable 30 (please see FIG. 3) for extending the distancebetween the proximity probe 12 and the system 10. The extension cable 30includes a first conductor 32 and a second conductor 34 extendingbetween a first end 36 and a second end 38 of the extension cable 30.The extension cable 30 is coupled between the probe cable 20 and thesystem 10. Particularly, the first end 36 of the extension cable 30 iscoupled to the second end 28 of the probe cable 20 via a cable coupling27 such that the conductor 22 is connected to conductor 32 and conductor24 is connected to conductor 34. The first conductor 32 at the secondend 38 of the cable 30 is coupled to the second terminal 42 of theresistance means 40 at node 46 and the second conductor 34 is coupled tothe ground node 48. Thus, the extension cable 30 is coupled in serieswith the probe cable 20.

[0091] The signal generator means 70 is operatively coupled to the firstterminal 41 of the resistance means 40 at node 44 for driving a signalthrough the resistance means 40, and the probe 12 thereby impressing thefirst voltage V₁ across the serially connected resistance means 40 andprobe 12, and impressing the second voltage V₂ only across the probe 12(and extension cable if employed). Preferably, the signal generatormeans 70 is operatively coupled to the resistance means 40 at node 44via the filter means 50 and to the digital signal processor for drivinga programmable dynamic signal of one or more frequencies through thefilter means 50 and the serial connection of the resistance means/probecombination.

[0092] In particular, the signal generator means 70 preferable includesa direct digital synthesis (DDS) device 72 operatively coupled to thefirst terminal 41 of the resistance means 40 via the filter means 50 andthe buffer, gain and offset means 60 for driving the dynamic signal orwaveform through the resistance means 40 and the probe 12 (and extensioncable if employed). This dynamic signal causes the first voltage V₁ tobe impressed across the serial connection of the resistance means 40 andprobe 12 and causes the second voltage V₂ to be impressed only acrossthe probe 12 (and extension cable if employed). Typically, the sensingelement 14 of the probe 12 is strategically coupled proximate the targetto be monitored such that this dynamic signal causes the sensing element14 of the probe 12 to generate an alternating magnetic field thatinduces eddy currents in the metallic target object T. In turn, the eddycurrents in the target induce a voltage in the sensing element 14 of theprobe 12 and hence, a change in an impedance of the probe which variesas a function of, inter alia, variations of spacing or gap between theprobe and the target being monitored.

[0093] The signal generator means 70 can include a plurality of DDSdevices 72 coupled to the first terminal 41 of the resistance means 40via the filter means 50 and the buffer, gain and offset means 60 fordriving a plurality of dynamic signals at different frequencies throughthe resistance means 40, the probe 12 and extension cable (if employed)and subsequently performing processing including convolution asdelineated in detail infra for obtaining simultaneous impedancemeasurements of the probe 12 at different frequencies which arecorrelative to the gap interposed between the probe 12 and the targetbeing monitored.

[0094] Each direct digital synthesis device 72 is preferably coupled tothe DSP means 110 via interface 114 and generates a purefrequency/phase-programmable dynamic signal such as a sinusoidal wave.The DSP means 110 preferably includes an algorithm to program both thefrequency and the phase of the output signals which in turn can be usedto drive the probe with a frequency/phase-programmable dynamic analogsignal having an output frequency/phase which can be preciselymanipulated under full digital control. Thus, each DDS device can bedigitally programmed to output sine waves at any frequency/phase withprecision for use as driving signals or reference signals. One exampleof the DDS devices 72 is that which is manufactured by Analog Devicesand sold under part number AD9850.

[0095] The filter means 50 is interposed between the DDS devices 72 andthe resistance means 40 for filtering the analog dynamic signals outputfrom DDS devices 72. The filter means 50 preferably includes at leastone low pass filter 52 interposed between each direct digital synthesisdevice 72 and the first terminal 41 of the resistance means 40 to purifythe output dynamic signals or waveforms of each synthesis device 72 foreliminating, inter alia, the harmonics that are created in the synthesisdevices 72. For example, as a result of the outputs of the directdigital synthesis devices 72 being ten plus bit digital to analogconverters, the quantitization noise needs to be filtered out using alow pass filter. Thus, the filters 52 remove the steps and smoothes outthe analog dynamic signal outputs from the DDS devices 72. Additionally,the filters 52 helps reduce the noise bandwidth of the system 10 whichimproves a signal to noise ratio. Furthermore, a half bit of noise canbe summed in at node 54 to change the quantitization noise using adithering process. Preferably the low pass filters 52 are in the form offive pole elliptical filter devices.

[0096] Buffer, gain and offset means 60 can be interposed between filtermeans 50 and the resistance means 40 for buffering and amplifying theanalog dynamic signals and providing any desired offset of same.

[0097] The sampling means 90 is coupled to the first node 44 forsampling and digitizing the first voltage V₁ impressed across theserially connected resistor/probe combination. Additionally, thesampling means 90 is coupled to the second node 46 for sampling anddigitizing the second voltage V₂ impressed only across the probe 12 (andextension cable if employed). Preferably, the sampling means 90 includesa pair of analog to digital converters 92, 94 coupled to the first node44 and the second node 46 respectively for sampling and digitizing thefirst dynamic voltage V₁ and the second dynamic voltage V₂. The analogto digital (A-D) converters 92, 94 are preferably 14 bit, wide bandwidthconverters manufactured by, for example, Analog Devices under partnumber AD9240.

[0098] The timing control means 80 provides the synchronization betweenthe output signal of the signal generator means 70 and the sampling rateof the sampling means 90 so that the phase relationship between theoutput signal and samples is maintained. The timing control means 80 isoperatively coupled to each DDS device 72, the pair of analog to digitalconverters 92, 94, and to the DSP means 110. Thus, the DDS devices 72are clocked by the timing control means 80 so that the frequency of theoutput of these devices is very accurately set. Additionally, the timingcontrol means 80 provides the synchronization between the output of theDDS devices 72 and the sampling rate of the analog to digital converters92, 94 so that the phase relationship between the dynamic drivingsignal(s) and the sampled signals is maintained. Thus, the sampling isperformed in synchrony with the dynamic driving signals. Note that thequartz clock oscillator 84 is operatively coupled to each DDS device 72for providing a clock signal thereto.

[0099] Specifically, the timing control means 80 is an agile-clockgenerator which preferably includes a DDS device 82 operatively coupledto and clocked by a quartz clock oscillator 84. An output of the DDSdevice 82 is preferable filtered by a five pole elliptical filter 86 andfeedback to the DDS device 82 which then outputs a triggering signal tothe analog to digital converters 92, 94 and a timing signal to the DSPmeans 110. The DSP means 110 is operatively coupled to the DDS devices72, 82 and may employ the timing signal sent to the DSP means 110 whendigitally programming the DDS devices 72, 82 to orchestrate thesynchronicity between the sampling rate and the dynamic signals outputby the DDS devices 72 to the probe 12. This assures that when thevoltages V_(1D) and V_(2D) are calculated there will be exactly onecycle worth of data stored per data set. Thus, the DDS devices can beused for generating the dynamic signals which excite the sensing element14 and for generating timing signals for triggering the sampling by thepair of analog to digital converters 92, 94.

[0100] The convolution means 100 can be a stand-alone device in the formof, for example, a digital down counter (DDC), that just doesconvolution. In this embodiment, the convolution means 100 is interposedbetween and coupled to the sampling means 90 and the DSP means 110 to dothe convolution operation. The analog to digital converted values (thedigitized voltage signals V_(1D), V_(2D) which represent the dynamicvoltages V₁ and V₂ at respective nodes 44 and 46) are received andconvolved by the convolution means 100 and then supplied to the DSPmeans 110 as complex voltage numbers V_(1C), V_(2C). The advantage ofthis embodiment is that it is no longer necessary to vary the samplerate to remain synchronized to the signal generator. The DDC has thecapability of being programmed for what frequency to process. Oneexample of a commercially available digital down counter (DDC) ismanufactured by Harris Semiconductor under part number HSP 50016.

[0101] Alternatively, the digital convolution means 100 can beintegrally formed with the digital signal processor means 110 whereinthe DSP means 110 is operatively coupled to the pair of analog todigital converters 92, 94 for receiving the first and second digitizedvoltage signals V_(1D), V_(2D) from the converters and convolving thedigitized voltages into respective complex voltage numbers V_(1C),V_(2C) via the integral convolution means 100. Examples of DSP means 110having integral convolution means 100 can be found in the 210XX seriesof devices manufactured by Analog Devices. Preferably, the DSP means 110is a 40-megahertz floating point device or faster.

[0102] The process of convolving the digitized voltages into respectivecomplex voltage numbers V_(1C), V_(2C) via the convolution means 100 isdefined as inphase and quadrature detection or quadrature synthesis.More specifically, and referring to FIG. 5, the digitized voltagesignals V_(1D), V_(2D) can be represented by the function Vcos(wT+phi)and the circles with the “X” in the middle represent digital multipliers102,104 integrally formed in the convolution means 100. The digitizedvoltage signals V_(1D), V_(2D) are each multiplied by a digital cosineand sine waveform (cos(wT) and sin(wT)) which can be pulled from a tablein memory 101, memory 112 or from memory means 120. The results of thosemultiples are accumulated and averaged or filtered by, for example, theconvolution means 100 or the digital processor means 110 to get DCcomponents. These filtered (averaged) values or transformed valuesrepresent the magnitude of real and imaginary components of the complexvoltages V_(1C), V_(2C) of the convolved digitized voltage signalsV_(1D), V_(2D). In other words, these DC components are the inphase andquadrature components of the digitized voltage signals V_(1D) and V_(2D)and represent magnitudes of real and imaginary components therebyforming complex voltage numbers V_(1C) and V_(2C) from the dynamicvoltage signals V₁ and V₂.

[0103] An alternative way of looking at this is that when you multiply asignal (e.g. either of the data strings coming out of the analog todigital converters tied to node 44 (V₁) or node 46 (V₂)) by a sine or acosine wave of the same frequency, you get a DC term and a 2X AC term.Averaging the output of the multiplication then filters out the AC term.When this multiplication is performed using both a cosine and a sinefunction, you get two DC terms that represent the inphase and quadraturecomponents of the signal. These are the real and the imaginary valuesfor voltage. Note, a scaling term is usually needed after the averagingto get back to other proper engineering units. However, the term cancelsout since voltage appears in both the numerator and denominator as aresult of the instant invention using the voltage ratio method definedhereinabove.

[0104] Another possible way to implement the convolution method ascontemplated by the instant invention is to interpose a FieldProgrammable Gate Array (FPGA) between the analog to digital convertersand the DSP means 110. The difference between this configuration andthat described hereinabove for the convolution device is that thestructure of the hardware necessary to perform convolution method isbuilt and programmed into the Field Programmable Gate Array (FPGA).

[0105] At this point it is important to note that the idea of usingdigital convolution means 100 is paramount because if the multiplierswhere to be of an analog design, the accuracy of the multipliers becomesa critical error source. For example, if one needs to distinguish avalue of 0.005 in something of magnitude 300+i100 then a requiredstability at 1 MHz would be on the order of 1 part in 63,000. This isone to two orders of magnitude better than you can get out of an analogmultiplier. Thus, the instant invention solves this problem by usinganalog to digital converters for sampling the voltage signals V₁ and V₂and then performing multiplication in digital format to handle thestability problem. Additionally, the instant invention employs highspeed analog to digital converters to avoid the additional error source(multiplier gain drift).

[0106] Once the complex voltage numbers V_(1C) and V_(2C) aredetermined, the digital signal processor means 110 processes the complexvoltage numbers into the unknown impedance of the probe by preferablyusing the voltage ratio equation:Z_(unknown)=[V_(2C)/(V_(1C)−V_(2C))]*R. The DSP means 100 cancontinuously accumulate, process and store data from the convolutionmeans 100 and output signals as a function of the calculated impedancewhich are correlative to the gap between the probe and the target (e.g.,a rotating shaft of a machine or an outer race of a rolling elementbearing) being monitored. Particularly, the calculated impedance can beconverted by the processor 110 into a voltage or gap value correlativeto the spacing or gap between the probe and target being monitored byusing equation(s), algorithms, numerical methods or lookup tables storedin, for example, the memory means 120. It is important to note that thevoltage ratio alone can be used to determine values representative ofgap by accounting for the known resistance value within the equation(s),algorithms, numerical methods or look up tables.

[0107] Moreover, the digital signal processor can apply digital signalsto the signal generator means 70 for digitally reprogramming, in realtime, the generator means 70 for driving the probe 12 at one or morefrequencies corrective of anomalies due to, for example: temperaturevariations; changes in the conductivity and permeability in the target,proximity probe (including the integral sensing coil and probe cable)and extension cable, and anomalies due to phase shift ascertained fromthe measured impedance values.

[0108] In addition to the unique voltage ratio method, the digitalproximity system 10 includes a unique resonant method which emulates theoperation of analog eddy current proximity systems thereby providingboth backwards and forwards compatibility with existing and futureanalog systems. Thus, the digital proximity system 10 provides a directone for one replacement of existing and future analog eddy currentproximity systems.

[0109] For background, and as delineated in the background of theinvention, a typical analog eddy current proximity system includes aproximity probe located proximate a metallic target object (e.g., arotating shaft of a machine or an outer race of a rolling elementbearing) being monitored, an extension cable (if employed) and analogconditioning circuitry which includes a resonate circuit. The target,probe, extension cable and conditioning circuitry are all designed tointeract in such a way that a voltage output from the circuitry isdirectly proportional to a distance from the probe to the target andthis distance is commonly referred to as “gap”.

[0110] In general, and referring to FIG. 9, the resonant method includesthe steps of measuring the impedance of the probe and the extensioncable (if employed), mirroring the impedance of analog proximitycircuitry, computing a combination of the mirrored impedance with themeasured probe and extension cable impedance (if employed), anddetermining a gap value as a function of the computed impedance which iscorrelative to the spacing or gap interposed between the probe and ametallic target being monitored. It should be understood that if theextension cable is not employed the system 10 would then only measurethe impedance of the probe and compute the combination of the mirroredimpedance with that of the measured probe impedance.

[0111] In particular, and referring to FIGS. 1 through 4, the digitalProximity system 10 uses the unique voltage ratio apparatus and methodas delineated in detail hereinabove to first determine a complex numberthat represents the complex impedance of any probe and extension cable(if employed) that is new or that was previously coupled to an analogconditioning circuitry input. The complex number that represents thecomplex impedance of the probe and extension cable can be held in amemory such as the memory of the DSP means 110 and/or the memory means120 which may or may not be integral with the DSP means 110.

[0112] Next, the system 10 mimics the loading that an analogconditioning circuit puts on a tank impedance of an included resonatecircuit. The system 10 does this by putting a mathematical inductorand/or capacitor (mathematical Z) in parallel with the probe andextension cable impedance (please see FIG. 3). Therefore, the system 10accurately mimics what happens in an actual analog conditioningcircuitry for accomplishing the important task of providing backwardscompatibility with the existing, field installed, transducers.

[0113] One way of explaining how the system 10 mirrors or mimics theanalog conditioning circuitry is by example. The schematic shown in FIG.6 shows one example of an analog conditioning circuit 170 having aresonate circuit and those having ordinary skill in the art and informedby the present disclosure should recognize the following:

[0114] First, that there is considerable circuitry in the analogconditioning circuitry that is in parallel with the impedance of theprobe and/or extension cable at connector J2 which affects the magnitudeof the impedance and the frequency of operation.

[0115] Second, that the current driven into the tank impedance at nodeN_(Zp) is supplied from a collector of Q1.

[0116] Third, that the voltage at the collector of Q1 will be the tankimpedance times the current supplied or in other words, the voltage atthe collector of Q1 is directly proportional to the magnitude of thetank impedance that is to be measured. Thus, the actual value for thetank impedance (Zp) will be determined at the collector of Q1.

[0117] Fourth, that there is a transfer function from the collector ofQ1 to a typical amplitude detector 172 in analog eddy current proximitysystems.

[0118] Fifth, that there is a transfer function from the collector of Q1to the feedback of the oscillator and that this feedback affects thefrequency of operation.

[0119] Therefore, according to the instant invention, one method formirroring or accurately mimicking analog circuitry in the digitalproximity system 10 includes determining an equivalent impedance of theanalog conditioning circuitry, for example the equivalent impedance ofthe analog conditioning circuitry 170.

[0120] The equivalent impedance may be determined by, for example,determining the equivalent circuit of the analog conditioning circuitry,pre-computing the impedance at different frequencies and using a look uptable (stored in memory 112 or memory means 120) to grab the appropriateimpedance value or any other method of determining circuit impedancewhich is known in the art. The first method provides the convenience ofallowing one to verify a mathematical model versus what the systemreally does while the second method is more computationally efficient.Note that empirical testing may be required to match the actual systemresponse to the mathematical model.

[0121] For example, FIG. 7 shows a thevenin equivalent circuit 174 ofthat which is shown in FIG. 6. Some of the components therein havenegligible contribution to the overall impedance, but it is preferred toadd them for completeness. Some other impedances that may have a minorimpact on computing Zp are, for example, the base impedance of thetransistors.

[0122] Therefore, according to the instant invention, the combination ofthe impedance of the probe and extension cable (if employed) and theequivalent impedance of the analog conditioning circuitry 170 is thencomputed. This computed value is the tank impedance (Zp) and iscorrelated by the system 10 to a gap value by using equation(s),numerical methods, algorithmic functions or lookup tables whereinimpedance values are correlated to gap values defining the gap orspacing interposed between the probe and the target being monitored.This method of measuring gap can be continuously repeated formonitoring, for example the vibration of a rotating shaft of a machineor an outer race of a rolling element bearing.

[0123] Notwithstanding, one design question that should be answered iswhether the analog system is at the right frequency. The voltagedeveloped at the collector of Q1 is assumed to be directly proportionalto the tank impedance so it is exactly in phase with the voltagedeveloped. This voltage is feedback to the base of Q2 to make the analogsystem oscillate, so it goes through the feedback network 176 shown inFIG. 8.

[0124] The phase shift from the node marked N_(Zp) to the feedbackvoltage node N_(Fv) is the phase delay of the oscillator. This phaseshift is added to the phase of the tank impedance and should equal thephase of the oscillator under steady-state operation. If there is aninequality, the oscillator is not at steady state and will slew towardsthe frequency that satisfies that relationship. To account for this thedigital proximity system 10 computes a phase error which is defined bythe following equation: Phase error=phase the oscillator runs at(usually 0 to 6 degrees)−Phase of the tank impedance+the phase delay inthe feedback network. Preferably, the digital proximity system 10multiplies this calculated phase error by a pre-computed gain term tocompute how far to adjust the frequency to mimic steady state operationof the analog circuitry. This calculation can be computed in the DSPmeans 110. Thus, a digital feedback loop including the signal generatormeans 70 can receive digital feedback signals from the digital signalprocessor means 110 which are correlative to any anomalous phase errorascertained from the measured impedance for digitally reprogramming, inreal time, the generator means 70 for driving the sensing coil 14 of theprobe 12 which adjusts the frequency to mimic steady state operation ofthe analog circuitry.

[0125] Thus, the digital proximity system 10 may have to account for thefrequency shift before it can compute the gap of the system.

[0126] Typically, the components used in the analog conditioningcircuitry are called out on bill of materials (BOMs) and installed intothe printed wiring assemblies (PWAs). The Zp versus gap is set when theresistors are tweaked during calibration of the analog conditioningcircuitry. In the digital Proximity system 10, these values aremathematical constructs stored in a file in the memory means 120 andpulled out as needed to work with whatever device the system 10 happensto be plugged into at the time.

[0127] More specifically, and referring to FIG. 10, the resonant methodincludes measuring the unknown impedance or an uncompensated impedanceof the probe located proximate the target T and an extension cable 30(if employed) as delineated in detail supra. Next, compensation factorsor coefficients are determined from open/short or open/short/loadcalibration tables 125 stored in memory 112 or memory means 120 formedfrom an open/short or open/short/load calibration or compensation methodwhich will be described in detail infra. The measured impedance is thencompensated by using the coefficients determined from the open/short oropen/short/load calibration tables 125. The equivalent or load impedancevalue that the analog proximity circuitry would have had in parallelwith the probe and extension cable is mathematically combined with thecompensated impedance to form the “tank impedance” of the system 10. Asnoted hereinabove, the equivalent impedance values can be determined by,for example, using lookup tables 123 of empirically determined values orof mathematically modeled values. The calculated tank impedance is thenused to determine the gap the system is at. Alternatively, both thecurrent frequency and the calculated tank impedance can be used todetermine the gap the system is at by using one or more mathematicalequations 121, one or more look up tables 123, numerical methods 122 orvia algorithmic functions 124. Next, the calculated tank impedance canbe used to determine the phase shift that would have be needed to befeedback to the oscillator of the analog proximity circuitry so that itheads towards its final steady state frequency setting. This phase shiftcan be used to adjust the frequency of the dynamic signal of the system10 which is driving the probe 12. These steps or a subset of these stepsare iteratively repeated to continuously monitor the gap between theprobe and the target being monitored.

[0128] For example, and referring to FIG. 10, the resonant method caninclude the steps of measuring the uncompensated impedance of the probelocated proximate the target T and an extension cable 30 (if employed)as delineated in detail supra. Then, determining compensationcoefficients from the open/short/load calibration tables 125 stored inmemory 112 or memory means 120, compensating the measured impedance byusing the determined coefficients and determining a gap value as afunction of the compensated impedance.

[0129] In another example, the unknown impedance of the probe and/or theextension cable can be measured at one fixed frequency to allow for thesystem 10 to be simplified. The following steps outline one methodaccording to the instant invention for measuring an unknown impedanceZ_(unknown) when using a single fixed frequency. The steps include:measuring the uncompensated impedance of the probe and extension cable(if employed); compensating the measured impedance using cablecoefficients appropriate for the single fixed frequency; determining agap value as a function of the compensated measured impedance, anditeratively repeating the measuring, compensating and determining stepsto substantially continuously measure the gap between the probe and thetarget being monitored for providing data correlative to machine status.

[0130] The coefficients appropriate for the measured impedance at thefixed single frequency can be empirically predetermined using, forexample, the open/short or open/short/load calibration method and thenstored in memory 112 or memory means 120 and then later recalled for usewith the measured impedance value for determining the gap value as afunction of the compensated measured impedance of the probe and/orcable. Furthermore, the equivalent or load impedance that the analogproximity circuitry would have had in parallel with the probe andextension cable can be combined with the compensated impedance and usedin determining the gap value as a function of the combined impedance.

[0131] Note that each time the gap is measured using, inter alia, theresonant method it can be output in analog or digital form.

[0132] The cable calibration or compensation methods referred to suprawill now be explored.

[0133] In the environment of machine monitoring the unknown impedancemeasured by the digital proximity system 10 can be that of the extensioncable and the proximity probe including the integral sensing element 14and probe cable 20. The instant invention includes an open/shortcompensation method and/or an open/short/load compensation method thatcan be employed for eliminating cable residuals (residual cableimpedance and stray cable admittance) of either the extension or probecable, or both. These two methods and the differences therebetween willbe described with the assistance of FIGS. 11 through 14 and then adetailed delineation will be presented on how these methods can beemployed for eliminating cable residuals in order to obtain true orcompensated probe impedance (Z_(probe)) and true or compensated sensingcoil impedance (Z_(sensing element)). These measured impedances orcompensated impedances can then be correlated by the system 10 to a gapvalue by using equations 121, numerical methods 122, algorithmicfunctions 124 or lookup tables 123 wherein gap values are correlated tomeasured or compensated impedance values defining the gap or spacinginterposed between the probe and the target being monitored. This methodof measuring gap can be continuously repeated for monitoring, forexample the vibration of a rotating shaft of a machine or an outer raceof a rolling element bearing.

[0134] In general, and referring to FIGS. 11 through 13, the open/shortcalibration method models the residuals (e.g., residual impedance andstray admittance) as a linear two port or four terminal networkrepresented by ABCD parameters. It should be noted that the open/shortcalibration method assumes that the network is a symmetrical network.From this restriction, the open/short compensation does not require aload measurement to know each value of the ABCD parameters. Referring toFIG. 11, a theoretical explanation and procedure are as follows:$\begin{bmatrix}V_{1} \\I_{1}\end{bmatrix} = {{\begin{bmatrix}{A\quad B} \\{C\quad D}\end{bmatrix}\begin{bmatrix}V_{2} \\I_{2}\end{bmatrix}} = \begin{bmatrix}{{A\quad V_{2}} + {B\quad I_{2}}} \\{{C\quad V_{2}} + {D\quad I_{2}}}\end{bmatrix}}$ therefore; V₁ = A  V₂ + B  I₂ I₁ = C  V₂ + D  I₂

[0135] thus, the measured impedance Z is represented as: $\begin{matrix}{Z = {\frac{V_{1}}{I_{1}} = {\frac{{A\quad V_{2}} + {B\quad I_{2}}}{{C\quad V_{2}} + {D\quad I_{2}}}.}}} & \left( {{equation}\quad 1} \right)\end{matrix}$

[0136] Open measurement: when the unknown terminals 47, 49 are opened,I₂=0. Then, from equation 1, the measured impedance Z_(OM) is:

Z_(OM)=A/C  (equation 2).

[0137] Short measurement: when the unknown terminals 47, 49 are shorted,V₂=0. Then, from equation 1, measured impedance Z_(S) is:

Z_(S)=B/D  (equation 3).

[0138] A limited condition for the ABCD parameters is as follows: whenthe unknown network is “symmetric” A=D (equation 4). Thus, a symmetricalnetwork can be defined as one where the parameters A and D are equal toone another.

[0139] DUT (Device Under Test) measurement: when the DUT is connected toterminals 47, 49, its impedance value is represented as Z_(X)=V₂/I₂(equation 5).

[0140] From equations 1 and 5, its measured impedance value Z_(XM) is:$\begin{matrix}{Z_{X\quad M} = {\frac{V_{1}}{I_{1}} = {\frac{{A\quad V_{2}} + {B\quad I_{2}}}{{C\quad V_{2}} + {D\quad I_{2}}} = {\frac{\frac{A\quad V_{2}}{I_{2}} + B}{\frac{C\quad V_{2}}{I_{2}} + D} = {\frac{{A\quad Z_{X}} + B}{{C\quad Z_{X}} + D}.}}}}} & {{equation}\quad 6}\end{matrix}$

[0141] Equation 6 can be solved for Z_(X) and from equations 2 and 3, Aand B are erased to give: $\begin{matrix}{Z_{X} = {\frac{D}{C}{\frac{Z_{S} - Z_{X\quad M}}{Z_{X\quad M} - Z_{O\quad M}}.}}} & {{equation}\quad 7}\end{matrix}$

[0142] From equations 2 and 4, unknown parameters C and D in equation 7can be erased and then the true or compensated value of the unknownimpedance of the DUT can be defined as Z_(X) which is determined by anopen/short equation which is as follows: $\begin{matrix}{Z_{X} = {Z_{O\quad M}\frac{Z_{S} - Z_{X\quad M}}{Z_{X\quad M} - Z_{O\quad M}}}} & {{equation}\quad 8}\end{matrix}$

[0143] wherein:

[0144] Z_(OM) is the measured open impedance

[0145] Z_(S) is the measured short impedance

[0146] Z_(XM) is the measured value of the unknown impedance Z_(uknown)of the DUT Note: all values are complex numbers.

[0147] In general, the system 10 can employ the open/short method asfollows. First, the impedance of the probe cable 20, the extension cable30 or both coupled together is measured with one end left opened (FIG.12) for defining Z_(OM). Second, the impedance of the probe cable 20,the extension cable 30 or both coupled together is measured by shortingthe one end (FIG. 13) for defining Z_(S). Third, an unknown impedance iscoupled to the probe cable 20, the extension cable 30 or both and itsmeasured impedance Z_(XM) is obtained by preferable using the voltageratio apparatus and method of the system 10 as delineated supra whereinZ_(XM) is the Z_(unknown) described hereinabove. Finally, the true orcompensated value Z_(X) of the measured unknown impedance is determinedfrom the open/short equationZ_(X)=Z_(OM)*(Z_(S)−Z_(XM))/(Z_(XM)−Z_(OM)).

[0148] For example, the true or compensated value of the impedance ofthe proximity probe 12 coupled to the system 10 via the extension cable30 can be calculated from the following equation:

Z_(X)=Z_(OM)*(Z_(S)−Z_(XM))/(Z_(XM)−Z_(OM))

[0149] wherein:

[0150] Z_(OM) and Z_(S) are respectively, the measured open and shortimpedances utilizing only the extension cable 30,

[0151] Z_(XM) is the measured impedance of the proximity probe 12coupled to the extension cable 30 which in turn is coupled to the system10 (Note that in this example Z_(XM) is the Z_(unknown) described suprawith the extension cable employed), and

[0152] Z_(X) is the true or compensated impedance value of the proximityprobe 12 wherein the residuals of the extension cable 30 are eliminated.

[0153] Thus, the extension cable residuals are mathematically eliminatedby using the compensation coefficients Z_(OM), Z_(S) and Z_(XM) todefine the compensated proximity probe impedance of the proximity probe12.

[0154] Likewise, Z_(OM) and Z_(S) can be respectively, the measured openand short impedances utilizing only the probe cable 20 and Z_(XM) can bethe measured impedance of the sensing element 14 coupled to the probecable 20 which in turn is coupled to the system 10. Thus, Z_(X) wouldthen be the true or compensated value of the sensing element 14 with theresiduals of probe cable 20 eliminated.

[0155] Additionally, Z_(OM) and Z_(S) can be respectively, the measuredopen and short impedances utilizing both the probe cable 20 and theextension cable 30 coupled together and Z_(XM) can be the measuredimpedance of the sensing element 14 coupled to the probe cable 20 whichin turn is coupled to the system 10 via the extension cable 30. Thus,Z_(X) would then be the true or compensated value of the sensing element14 with the residuals of both the probe cable 20 and the extension cable30 eliminated.

[0156] Thus, the residuals of the probe cable 20, the extension cable 30or both coupled together can be mathematically eliminated by using thecompensation coefficients Z_(OM), Z_(S) and Z_(XM) to define thecompensated proximity probe impedance of the proximity probe 12 or thecompensated sensing element impedance of the sensing element 14.

[0157] Furthermore, true impedance values or compensation coefficientsfor different extension or probe cable configurations includingdifferent lengths can be stored in memory, for example, memory means 120as look up or calibration tables 125 and recalled when necessary. Forexample, the system 10 can measure the impedance of the serial couplingof the extension cable 30 and proximity probe 12 and then eliminate viaimpedance values or compensation coefficients the extension cableimpedance to determine the proximity probe impedance or eliminate viaimpedance values or compensation coefficients both the extension cableimpedance and probe cable impedance to determine the sensing element orcoil impedance. Likewise, the system 10 can measure the impedance of theproximity probe 12 and then eliminate via impedance values orcompensation coefficients the probe cable impedance to determine thesensing element or coil impedance.

[0158] The following three methods can be used to determine the valuesfor Z_(OM) and Z_(S). First, a tuned cable length is used where thelength is set so that they most closely match known compensation valuesstored in the memory means of the system 10. Second, a user installs theextension cable to an input of the system 10 and then performs theopen/short compensation method as described above and then stores themeasured values in the memory means 120. Third, by mathematicallydetermining the cable length and compensation values and using thesevalues to eliminate cable residuals.

[0159] The DDS devices of the system 10 can be used for generating thedrive signal by loading digital control signals from the DSP into theDDS and letting the DDS drive the probe cable 20, the extension cable 30or both with arbitrary waveforms at whatever frequency is needed.

[0160] The open/short/load compensation is an advanced compensationtechnique that is applicable to complicated residual circuits and is thepreferred cable calibration or compensation method according to theinstant invention. To perform open/short/load compensation, threemeasurements are required before measuring a DUT (e.g., the unknownimpedance of the probe monitoring a metallic target object). Thesemeasurements include a measurement with the unknown terminals 47, 49open, a measurement with the unknown terminals 47, 49 shorted and ameasurement with a standard DUT (load) having a known value coupledbetween the unknown terminals 47, 49.

[0161] The open/short/load compensation method is particularly usefulwhen the extension cable 30 is used whose length cannot be compensatedwith a cable length correction function nor be minimized with theopen/short correction method described above.

[0162] The open/short/load correction requires the measurement data ofat least one standard DUT having a known value in addition to theopen/short measurement data. Similarly to the open/short method, andreferring to FIG. 11, the open/short/load compensation method models theresiduals as a four-terminal network circuit represented by the ABCDparameters. Each parameter is known if three conditions are known and ifthe four-terminal network is a linear circuit.

[0163] From FIG. 11,${Z_{1} = {V_{1}/I_{1}}},{Z_{2} = {{{V_{2}/I_{2}}\quad a\quad n\quad d\quad Z_{1}} = {\frac{{A\quad V_{2}} + {B\quad I_{2}}}{{C\quad V_{2}} + {D\quad I_{2}}} = \frac{{A\quad Z_{2}} + B}{{C\quad Z_{2}} + D}}}}$

[0164] The parameters A, B, C and D can be removed when using thefollowing definitions:

[0165] Z_(O) is the measured open impedance with the terminals 47, 49open,

[0166] Z_(S) is the measured short impedance with the terminals 47, 49closed,

[0167] Z_(SM) is the measured impedance of the standard DUT whenconnected to the terminals 47, 49,

[0168] Z_(std) is the true (or expected) value of the standard DUT,

[0169] Z_(XM) is the measured value of the DUT having the unknownimpedance and connected to the terminals 47, 49 (for example, theproximity probe 12)

[0170] Z_(DUT) is the true or compensated value of the DUT having theunknown impedance (for example, the compensated impedance of theproximity probe 12).

[0171] The true or compensated value of the unknown impedance Z_(DUT) isdetermined by an open/short/load equation which is as follows:$Z_{D\quad U\quad T} = \frac{{Z_{s\quad t\quad d}\left( {Z_{O} - Z_{S\quad M}} \right)}\left( {Z_{X\quad M} - Z_{S}} \right)}{\left( {Z_{S\quad M} - Z_{S}} \right)\left( {Z_{O} - Z_{X\quad M}} \right)}$

[0172] Note: all values are complex numbers.

[0173] Thus, in general, the system 10 can employ the open/short/loadmethod as follows. First, an impedance of the probe cable 20, theextension cable 30 or both cables coupled together is measured with oneend left opened (FIG. 12) and is defined as Z_(O). Second, an impedanceof the probe cable 20, the extension cable 30 or both cables coupledtogether is measured by shorting the one end (FIG. 13) and is defined asZ_(S). Third, Z_(SM) is defined as impedance which is measured with aknown load (a standard DUT or load) coupled to one end of the probecable 20, to one end of the extension cable 30 or to one end of coupledcables (FIG. 14). Fourth, an impedance is coupled to the probe cable 20,to the extension cable 30 or to the combination of coupled cables andits impedance is measured using the voltage ratio apparatus and methodof the system 10 as delineated hereinabove for defining Z_(XM). Finally,the true or compensated impedance of Z_(XM) is then calculated from theopen/short/load equation delineated hereinabove.

[0174] For example, the system 10 can employ the open/short/load methodas follows. First, an impedance of the extension cable 30 is measuredwith one end left opened (FIG. 12) for defining Z_(O). Second, animpedance of the extension cable is measured by shorting the one end(FIG. 13) for defining Z_(S). Third, an impedance of the extension cableis measured with one end coupled to a known load (a standard DUT orload) as shown in FIG. 14 for defining Z_(SM). Fourth, the system 10 iscoupled to the extension cable 30 which in turn is coupled to theproximity probe 12 for measuring Z_(XM). Note that Z_(XM) is the samemeasurement as that of the measurement of the unknown impedanceZ_(unknown) delineated supra with the extension cable employed. Finally,the true proximity probe impedance (Z_(probe)) can then be calculatedfrom the open/short/load equation as follows:$Z_{probe} = \frac{{Z_{s\quad t\quad d}\left( {Z_{O} - Z_{S\quad M}} \right)}\left( {Z_{X\quad M} - Z_{S}} \right)}{\left( {Z_{S\quad M} - Z_{S}} \right)\left( {Z_{O} - Z_{X\quad M}} \right)}$

[0175] where:

[0176] Z_(O) is the measured open impedance of the extension cable 30(FIG. 12),

[0177] Z_(S) is the measured short impedance of the extension cable 30(FIG. 13),

[0178] Z_(SM) is the measured impedance of the load (standard load orstandard DUT) coupled to the extension cable 30 in place of theproximity probe 12 (FIG. 14),

[0179] Z_(std) is the known value of the load impedance (standard loador standard DUT),

[0180] Z_(XM) is the measured impedance of the probe 12 coupled to theextension cable 30,

[0181] Z_(probe) is the true or compensated complex electrical impedanceof the proximity probe 12 with the residuals of the extension cableeliminated.

[0182] Likewise, Z_(O) and Z_(S) can be respectively, the measured openand short impedances utilizing only the probe cable 20. The impedance ofthe probe cable can then be measured with a standard load coupled to oneend to define Z_(SM). Next, an impedance of the probe cable can bemeasured with the sensing element or coil 14 coupled to one end todefine Z_(XM). Note that Z_(XM) is the same measurement as that of themeasurement of the unknown impedance Z_(unknown) of the proximity probe12 delineated supra without the extension cable 30 employed. Finally,the true or compensated sensing element impedance (Z_(sensing element))can then be calculated from the open/short/load equation with theresiduals of probe cable 20 eliminated.

[0183] Additionally, Z_(O) and Z_(S) can be respectively, the measuredopen and short impedances utilizing both the probe cable 20 and theextension cable 30 coupled to one another and then, the impedance of thecoupled probe cable 20 and extension cable 30 is measured with astandard load coupled to the end of the probe cable 20 to define Z_(SM).Next, the impedance Z_(XM) can be determined by measuring the impedanceof the extension cable 30 coupled to the probe cable 20 which in turn iscoupled to the sensing element or coil 14. Thus Z_(XM) is themeasurement of the unknown impedance Z_(unknown) as delineated suprawith the extension cable 30 employed. Finally, the true or compensatedsensing element impedance (Z_(sensing element)) can then be calculatedfrom the open/short/load equation with residuals of both the probe cable20 and the extension cable 30 eliminated.

[0184] One specific method of canceling out the effects of an addedextension cable during the digital impedance measurement by the system10 using the open/short/load compensation method is as follows:

[0185] 1. A user cuts an extension cable 30 to the length that is mostbeneficial for the mechanical installation of the proximity probe 12 andinstalls a connector on the trimmed end.

[0186] 2. The user connects the trimmed extension cable 30 to the system10 at nodes 46, 48 with the probe end open.

[0187] 3. The user performs an open/short/load calibration at terminals47, 49 as described above on the trimmed extension cable 30 and the datais either manually or automatically stored in, for example, memory 120.

[0188] 4. The user connects the proximity probel 12, for example, a 0.5or 1.0 meter proximity probe 12 to the trimmed extension cable viaconnectors.

[0189] 5. The system 10 than measures the impedance and mathematicallyeliminates the residuals of the trimmed extension cable as describedsupra. The residual impedance is that of the probe 12. Thus, the system10 is calibrated to operate with the proximity probe 12 and outputs alinearized gap signal.

[0190] In another embodiment the system can employ a load/load/loadmethod as follows. First, an impedance Z_(L1) of the extension cable ismeasured with one end coupled to a first load. Second, an impedanceZ_(L2) of the extension cable is measured with one end coupled to asecond load. Third, an impedance Z_(L3) of the extension cable ismeasured with one end coupled to a third load. Fourth, an impedanceZ_(XM) is measured by the system 10 wherein the extension cable iscoupled between the digital proximity system 10 and the proximity probe12 and thus the system measures the impedance of both the proximityprobe 12 and the extension cable 30. Finally, the true or compensatedprobe impedance (Z_(probe)) can then be calculated from the followingequation:$Z_{probe} = \frac{{Z_{s\quad t\quad d}\left( {Z_{L1} - Z_{L3}} \right)}\left( {Z_{X\quad M} - Z_{L2}} \right)}{\left( {Z_{L3} - Z_{L2}} \right)\left( {Z_{L1} - Z_{X\quad M}} \right)}$

[0191] wherein:

[0192] Z_(L1) is the measured impedance of the extension cable 30coupled to a first load,

[0193] Z_(L2) is the measured impedance of the extension cable 30coupled to a second load,

[0194] Z_(L3) is the measured impedance of the extension cable 30coupled to a third load (standard load or standard DUT),

[0195] Z_(std) is the known value of the third load impedance (standardload or standard DUT),

[0196] Z_(XM) is the measured impedance of the coupled probe 12 andextension cable 30,

[0197] Z_(probe) is the compensated complex electrical impedance of theproximity probe 12.

[0198] Thus, the proximity probe impedance of the proximity probe iscalculated as a function of the measured impedance, the first loadimpedance, the second load impedance, and the third load impedance forcompensating for extension cable residuals. The proximity probeimpedance is then correlated to a gap between the proximity probe andthe conductive target material.

[0199] Preferably, the second load has an impedance that is less thanthe impedance of the first load and the third load has an impedance thatis less than the impedance of the first load and greater than theimpedance of the second load.

[0200] Additionally, note that this method can also be used to determinethe true value of the sensing element wherein Z_(L1), Z_(L2), Z_(L3)respectively replace Z_(O), Z_(S) and Z_(SM).

[0201] It should also be noted that the following conditions may berequired for the open/short and/or the open/short/load methods. First,when getting the open correction data, the distance between measurementterminals should be the same as the distance that is required foractually holding the DUT. Secondly, when getting the short correctiondata, the measurement terminals should be shorted or connected to ashorting device and the residual impedance should be less than theimpedance value of the DUT. Thirdly, when selecting a standard DUT(load) for the load correction step there is no restriction that aninductor must be used for inductance measurement, or capacitor must beused for a capacitance measurement. Any device can be used if itsimpedance value is accurately known. It is important to use a stablestandard DUT not susceptible to influences of environment such astemperature or magnetic fields. From this viewpoint, capacitors orresistors are better suited than inductors which are more susceptible tothe environment. When measuring a DUT's various impedance values, a 100to 1,000 ohm resistor standard DUT (load) may provide the best results.When measuring a DUT of one impedance value, a standard DUT (load)having approximately the same impedance as that of the DUT to bemeasured may provide the best results. A 100 to 1,000 ohm resistorstandard DUT (load) may provide the best results for a DUT having anunknown impedance which is either very high or very low.

[0202] A standard DUT (load) may be measured using a direct connectionto nodes 46, 48 of the system 10 after performing the open/shortcorrection method at nodes 46, 48 to determine compensation coefficientsto be used in calculating the value of the standard load using equationeight (8) as delineated hereinabove. Additionally, It should be notedthat the open/short/load compensation method may be required to employ astable known load which is measured in the same way that the DUT will bemeasured and is of the same approximate value.

[0203] In conclusion, the system 10 can be calibrated at nodes 46, 48 byusing the open/short/load method thereby defining a first calibrationplane at nodes 46, 48 and including in memory 120 a correction orcompensation function or table of the internal impedance of the system.For example, the calibration tables 125 can include a correctionfunction or compensation including compensation coefficients whichcompensate for the resistance means 40 such that only the ratioV_(2C)/(V_(1C)−V_(2C)) of the complex voltages is required to determinethe unknown impedance value Z_(unknown) of the probe 12. Additionally,when an extension cable is coupled to nodes 46, 48 the system 10 can becalibrated at the end 36 of the extension cable 30 by using theopen/short/load method thereby defining a second calibration plane.Thus, the calibration tables 125 can include a correction orcompensation function which compensates for the extension cable suchthat the ratio V_(2C)/(V_(1C)-V_(2C)) of the complex voltages iscorrelative to unknown impedance value Z_(unknown) of the probe 12 andnot the extension cable 30 and probe 12 combination.

[0204] Additionally, the system 10 can be used to make all of the abovedelineated impedance measurements for the open/short method, theopen/short/load method and the load/load/load method and store thesevalues in memory.

[0205] Referring to FIGS. 16 through 22, the digital proximity system 10further includes a unique material identification and calibrationmethod, a unique material insensitivity method, and a unique inductiveratio method.

[0206] In order to understand how these unique methods work, one mustfirst understand a “Normalized Impedance Diagram”. Referring to FIG. 15,a normalized impedance diagram is shown and is comprised of amultiplicity of normalized impedance curves. This graph can be generatedby taking a proximity probe and measuring its impedance at differentfrequencies and at different gaps from, for example, a standard E 4140steel target. The lines 182 through 197 radiating outward (from the(0.0, 1.0) points) are gap lines. They represent the normalizedimpedance due to the target at a constant frequency as the gap ischanged from very close (the rightmost ends of the lines) to thefarthest gap (the 0.0, 1.0 point). These lines rotate clockwise alongarrow F as the frequency is increased. The arcs 200 through 208represent the impedance of the probe located at a fixed gap as thefrequency varies.

[0207] The method of obtaining this basic normalization is as follows:

[0208] 1. Measure a far gap impedance of the probe: Far gapimpedance=R₀+jwL₀.

[0209] 2. Measure an impedance of the probe near the target: Near gapimpedance=R+jwL.

[0210] 3. Determine a normalized impedance which is comprised of anormalized resistance term and a normalized reactance term as follows:

[0211] Normalized resistance=(R−R₀)/wL₀ and

[0212] Normalized reactance=wL/wL₀.

[0213] 4. Plot each point on a graph and connect the points done at thesame frequency.

[0214] 5. Connect the points done at the same gap thereby obtaining agraph as shown in FIG. 10.

[0215] Each target has its own characteristic normalized impedancediagram and it has been observed that the curves rotate clockwise as theconductivity and permeability of the target increase. Also, it has beenobserved that there is much more reactive change with gap than there isa resistive change as the conductivity and permeability of the targetincrease. The prior art systems are much more sensitive to resistancechanges than to reactive changes which makes the prior art systems moredifficult to calibrate for high conductivity materials as a result ofthere not being much resistance change to detect.

[0216] Note that the basic normalization method can be followed tomeasure the far gap and the near gap impedance of the probe incombination with the extension cable to obtain a “Normalized ImpedanceDiagram” of the probe/extension cable combination.

[0217] Additionally, one or more normalized impedance curves can begenerated by taking a probe and measuring its impedance at differentfrequencies and different gaps with different target materials andstoring this information in, for example, the memory means 120.

[0218] Now with the basic normalization method in mind, the uniquematerial identification and calibration method, the unique materialinsensitivity method and the unique inductive ratio method will bedelineated in detail according to the instant invention.

[0219] Referring to FIGS. 16 and 17, the unique material identificationand automatic calibration method of the instant invention allows thesystem 10 to identify a material that the system 10 is monitoring and toautomatically calibrate the system 10 for that material.

[0220] More specifically, and referring to FIG. 16, the materialidentification and automatic calibration method of the digital proximitysystem 10 includes the following steps. At the outset, the probe of thedigital proximity system 10 is located proximate the target material tobe identified and the system 10 measures the impedance of the probe asdelineated in detail supra. The system 10 then determines the normalizedimpedance value of the probe by using this measured impedance which maybe compensated according to the instant invention and the far gapimpedance of the probe which, for example, can be manually entered viainput means 148 or called up from memory means 120. Next, the normalizedimpedance value can be correlated to a point or impedance on apreviously stored normalized curve for a specific target material andprobe type combination. Once the correlation is found, the material ofthe target can be identified. Alternatively, a user can enter thematerial type into the system 10 via an input means 148 which would thencorrelate a stored normalized curve to the specific target material andprobe type combination.

[0221] Note a multiplicity of normalized impedance curves for amultiplicity of different targets and probe types may be stored in thememory means 120 as, for example, material identification tables 126.These tables can then be accessed at any time by the system 10 foridentifying the material the probe is monitoring and performing the selfcalibration process based on the identified material. These curves canbe previously generated by taking one or more probes and measuring theirimpedance values at different frequencies and different gaps withdifferent target materials and storing this information in, for example,the table 126 of the memory means 120.

[0222] Additionally, the memory means 120 can include calibration data134 which includes the parameters necessary for automaticallycalibrating the system 10 for the identified material. Therefore, oncethe material of the target is identified, the digital proximity system10 can pull the appropriate system calibration data out of the memorymeans 120 for automatically calibrating the system 10 for the identifiedmaterial and thus, generating gap data for monitoring the identifiedmaterial.

[0223]FIG. 17 shows an example of a normalized impedance diagram 210which reflects normalized impedance curves for the same probe running ata single frequency, but looking at multiple targets. Each line 212through 220 radiating outward from the (0.0, 1.0) point represents thenormalized impedance of different target materials at the constantfrequency as the gap is changed from very close (the rightmost end ofthe line) to the farthest gap (the 0.0, 1.0 point). This information canbe stored in memory 120 and the material identification and automaticcalibration method can be used to determine point 222 and correlate thepoint to a normalized curve of a 4140 type of material. This informationcan then be used to determine the calibration parameters from thecalibration data stored in the memory 120 for automatically calibratingthe system 10 and generating, for this example, a gap of 33 mils.

[0224] Note that the aforementioned material identification andautomatic calibration method can be followed with the probe beingreplaced with the probe/extension cable combination to obtain anormalized impedance value which is correlated to a point on apreviously stored normalized curve for a specific target material. Oncethe correlation is found, the material of the target can be identified.

[0225] This method has the huge advantage of being backwards compatiblewith analog proximity systems.

[0226] As mentioned hereinabove, and referring to FIGS. 16 and 17, theunique material identification method of the instant invention allowsthe system 10 to identify a material located proximate the proximityprobe 20. disposed adjacent one or more different yet known targetmaterials and driven at one or more different frequencies. The system 10then calculates a normalized impedance curve for each different targetmaterial at each different frequency and stores these curves as anequation, as an algorithmic function or as a database of values in amemory, for example, memory means 120. Thereafter, the system 10subsequently identifies unknown materials by first digitally measuringthe complex impedance of the probe 20 driven at one or more differentfrequencies and disposed adjacent an unknown material. Next, the system10 calculates the normalized impedance curve for the unknown material atthe one or more different frequencies by using an equation analgorithmic function or a database of values for the unknown material ateach driving frequency. The system 10 then compares the equation (oralgorithm) determined for the unknown material to one or more previouslydetermined equations (or algorithms) for known materials to obtain orinterpolate a match for identifying the unknown material. Alternatively,the system 10 compares one or more values in the database of values forthe unknown material at each driving frequency to one or more values inone or more previously determined databases of known materials to obtainor interpolate a match for identifying the unknown material. Note thatall measured impedances can be compensated by using the open/short oropen/short/load compensation methods according to the instant inventionprior to the probe or coil impedance being normalized.

[0227] In a second form, the system 10 identifies a unknown material bydepending primarily on the curve shape of the normalized impedanceresponse and the angle of the vector the normalized impedance responsesweeps out from the normalized resistance value 0.0 and the normalizedreactance value 1.0 to the actual normalized impedance value of thetarget at any particular frequency. Thus, material identification is notbased on the absolute position of any normalized impedance reading andas a result, lines do not have to overlap to indicate the same materialcharacteristics. Variations in “liftoff” or separation between the probeand the target may cause variations in the absolute position of thenormalized impedance measurement, but will not affect its curve orangular relationship with the origin.

[0228] When different materials have very similar conductivity thesystem 10 can measure the impedance characteristics of each target attwo or more different gaps to determine the “liftoff line” for eachmaterial. This assists in identifying the relative position of thematerial's normalized impedance response when plotted on a graph.

[0229] The system 10 can utilize these forms alone or in combinationwith the material identification and automatic calibration method formonitoring rotating and reciprocating machinery as explained hereinaboveand with reference to FIGS. 16 and 17.

[0230] Another use of the material identification methods or formsaccording to the instant invention is in the area of identifying and/orsorting coins and precious metals. For example, the system 10 candetermine the normalized impedance of a series of coins and/or preciousmetals (targets) placed adjacent the probe 12. The normalized impedancevalues can be recorded at several different frequencies. These valuescan then be compared or plotted with known standards 123 of coins and/orprecious metals for material identification and discrimination.

[0231] Material discrimination is based primarily on the curve shape ofthe normalized impedance response and the angle of the vector thenormalized impedance response sweeps out from the normalized resistancevalue 0.0 and the normalized reactance value 1.0 to the actualnormalized impedance value of the target(s) at any particular frequency.Thus, material discrimination is not based on the absolute position ofany normalized impedance reading and as a result, normalized curves donot have to overlap to indicate the same material characteristics.Variations in “liftoff” or distance between the probe and any target maycause variations in the absolute position of the normalized impedancemeasurement, but will not affect its curve or angular relationship withthe origin.

[0232] When different materials have very similar conductivity thesystem 10 can measure the impedance characteristics of each target attwo different gaps to determine the “liftoff line” for each material.This assists in identifying the relative position of the material'snormalized impedance response when, for example, plotted on a graph.

[0233] One empirical example of the discrimination method delineatedabove used the system 10 to preform eddy current analysis on differentmetal and coin types. The system 10 measured the impedance of thedifferent metal and coin types, normalized these impedances and plottedthe results. It was shown that a curve shape of the normalized impedanceresponse and an angle of the vector or curve shape of the normalizedimpedance response that sweeps out from the normalized resistance value0.0 and the normalized reactance value 1.0 for gold, gold coins, silver,silver coins and copper-nickel silver dollars was such that thediscrimination of one from the other was easily discernable with asingle impedance measurement. These materials were also easilydiscriminated against platinum and palladium.

[0234] In the case of distinguishing between platinum and palladium,different materials having very similar conductivity, the system 10measured the impedance characteristics of each target at two differentgaps to determine the “liftoff line” for each material. This assisted inidentifying the relative position of the material's normalized impedanceresponse when, for example, plotted on a graph.

[0235] Referring to FIGS. 18 and 19, The unique material insensitivemethod of the instant invention allows the system 10 to monitordifferent target materials without having to be re-calibrated for eachdifferent material thereby providing a material insensitive digitalproximity system 10.

[0236] More specifically, and referring to FIGS. 18, a graph is shown ofa normalized impedance diagram for the system 10 running at a singlefrequency, but looking at multiple targets. Each line (230 through 238)radiating outward from the (0.0, 1.0) point represents the normalizedimpedance of different target materials at a constant frequency as thegap is changed from very close (the rightmost end of the line) to thefarthest gap (the 0.0, 1.0 point). These lines rotate clockwise as theconductivity and permeability of the target increase. The arcuate lines240, 242 and 244 are a series of locus which connect the points on eachline (230 through 238) that are at the same gap. One or more normalizedimpedance curves each including a series of locus can be generated bytaking a probe and measuring its impedance at different frequencies anddifferent gaps with different target materials and storing thisinformation in, for example, a database or table(s) 128 of the memorymeans 120. Each locus can be represented by an equation(s) 129 ornumerical methods 130 which approximate the arcuate lines of constantgap. Thus, the system 10 is designed so that any impedance lying on thelocus of constant gap would output the same gap reading. No end userinteraction is necessary in this method.

[0237] In one form, and referring to FIGS. 18 and 19, the materialinsensitive method of the digital proximity system 10 includes the stepsof: determining a plurality of normalized impedance curves as delineatedsupra for different materials and preferably storing the curves in adatabase of the memory means; defining a series of locus lines on theimpedance curves that represent the same gap for the differentmaterials; measuring an impedance of the probe located proximate atarget material to be monitored, normalizing the measured probeimpedance and comparing the normalized probe impedance with the seriesof locus (the arcuate lines) stored in the database for determining agap locus that corresponds to the normalized impedance value of theprobe wherein the corresponding gap locus reveals a gap valuesubstantially correct for any target material being monitored therebyproviding a material insensitive digital proximity system 10.

[0238] In another form, the material insensitive method of the digitalproximity system 10 includes the steps of: determining a plurality ofnormalized impedance curves as delineated supra for different materialsand defining a series of locus lines on the impedance curves thatrepresent the same gap for the different materials; storing anequation(s) or numerical methods which approximate the arcuate locuslines in the memory means; measuring an impedance of the probe locatedproximate a target material to be monitored, normalizing the measuredprobe impedance and using the equations(s) or numerical methods fordetermining a gap locus that corresponds to the normalized impedancevalue of the probe wherein the corresponding gap locus reveals a gapvalue substantially correct for any target material being monitoredthereby providing a material insensitive digital proximity system 10.

[0239] Note that the material insensitive method described hereinabovecan be followed when employing a probe/extension cable combination inplace of the probe only.

[0240] Specifically, The step of measuring the impedance of the probelocated proximate a target in the two former material insensitivemethods can further include measuring the impedance of the probe and anextension cable wherein all the subsequent steps are carried out usingthis measured combination of impedance in place of just the probeimpedance and all the previous steps are carried out using aprobe/extension cable combination.

[0241] In yet another form, and the material insensitive method of thedigital proximity system 10 can include the step of mathematicallyestimating the sensing element or coil impedance of the probe byremoving any contribution of impedance from the integral probe cable andthe extension cable (if employed). Any contribution of impedance fromthe integral probe cable and the extension cable (if employed) can bedetermined from the open/short/load calibration method delineated above.

[0242] Referring to FIGS. 20 through 22, the unique inductive ratiomethod of the instant invention allows a normalized impedance curve tobe determined for a specific target without knowing the far gapimpedance of the probe coil and thus, without removing the probe from amachine being monitored.

[0243] As delineated supra, the far gap impedance is needed to actuallydetermine the normalized impedance of the probe and to develop thenormalized impedance curve for a specific target which in turn can beused to determine the gap between the probe and the target beingmonitored. There is a normalized impedance value for each target at eachfrequency and gap.

[0244] Experiments have shown that normalized impedance diagramsgenerated from different probes in the same series of transducers havevery little variation. This is because the normalizing process used togenerate the diagrams removes the variations caused by differences inresistance and inductance between the different coils. All that remainsis the probe geometry and the target material. Coil geometry is veryconsistent in regards to how the target interacts with the coil. Infact, water in a coil will cause very little error in the normalizedimpedance diagram until the probe gets so wet it's almost shorted out.

[0245] Unfortunately, one can not use this technique directly to measureprobe gap because it depends on knowing the far gap impedance of theprobe coil, which can not be determined without removing the coil from amachine being monitored by the probe. Accordingly, there is a need for amethod and apparatus which solves these problems.

[0246] Referring to FIGS. 20 through 22, the digital proximity system 10includes the unique inductive ratio method which is based on thenormalized impedance response, but is independent of the unknownvariables.

[0247] In general, and assuming that the probe is mounted in a machineat an unknown gap, an initial step of the inductive ratio method is tomeasure the impedance of the probe at two different frequencies f₁, andf₂ (please see FIG. 20). Thus, the impedance at f₁, is X1=(R1+jwL1) andthe impedance at f₂ is X2= (R2+jwL2). Next, the instant inventionassumes that the far gap impedance at f1 is r1+jwl1 and the far gapimpedance at f2 is r2+jwl2. Then, the normalized impedance is calculatedas follows: For X1: R1n = (R1 − r1)/w111 w1L1n = w1L1/w111 For X2: R2n =(R2 − r2)/w212 w2L2n = w2L2/w212

[0248] As noted above the resistance is unreliable and therefore, thefocus will be on the reactance measurement. In addition, it was notedsupra that the far gap is unknown and as a result, w1L1n and w2L2n cannot be calculated because l1 and l2, inductances at far gap, areunknown.

[0249] However, applicant has discovered that a function can be definedto remove the unknown variables. Specifically, applicant has discoveredthat if a function is defined to equal a normalized reactance at f₁divided by a normalized reactance at f₂ the unknowns can be made todisappear. This function can be defined as, for example, an inductiveratio function γ. Therefore γ=(w1L1n/w2L2n) and from the equations forX1 and X2 hereinabove we have the following:

(w1L1n/w2L2n)=(w1L1/w1l1)/(w2L2/w212)

[0250] and thus,

(w1L1n/w2L2n)=(w1L1/w212)*(w2/w1)*(12/11),

[0251] wherein the first term on the right side of the equation can bemeasured by the system 10 or an impedance meter; the second term is theknown frequencies and the third term is the inductances at far gap whichchanges very little over frequency and therefore can be approximated asequaling one.

[0252] This function corresponds to using the change in inductancebetween two frequencies at one gap to determine the actual gap. A curve240 of the function γ versus gap can be precomputed and the valuedetermined in the measurement above can be used to estimate the gap ofthe probe in question. Graphically, this is shown in FIG. 21 wherein γwhich is depicted as γ(g) is plotted versus gap. In other words, the gapis equal to the measured reactance at f1 divided by measured reactanceat f2 times the frequency at f2 divided by the frequency at f1.Therefore, the instant invention provides a method (generally depictedin FIG. 22) that defines a function of gap that is primarily a functionof probe geometry without it's actual inductance or resistance being amajor factor.

[0253] Moreover, the inductive ratio method can further include the stepof mathematically removing the effect of the integral cable of the probeon the impedance of the sensing coil, mathematically removing the effectof the extension cable on the impedance of the probe, or mathematicallyremoving the effect of both the integral cable of the probe and theextension cable on the impedance of the sensing coil using the cablecalibration methods described hereinabove.

[0254] For example, the inductive ratio method can further include thestep of mathematically removing the effect of both the integral cable ofthe probe and the extension cable on the impedance of the sensing coil(using the cable calibration methods described hereinabove) to obtainonly the impedance of the coil after initially measuring the impedanceof the probe at two different frequencies f₁ and f₂. As a result, theimpedance at f₁ defined as X1=(R1+jwL1) and the impedance at f₂ definedas X2=(R2+jwL2) hereinabove can also be used to define the impedances ofonly the sensing coil at two different frequencies. Thus, the methoddefined supra can be identically carried out with the addition of thiscable compensation step.

[0255] Specifically, the inductive ratio method can include thefollowing steps. First, measuring the uncompensated impedance of theprobe and extension cable (if employed) at two different frequencies f₁and f₂. Second, determining compensation factors from open/short/loadcalibration tables. Since this method can be generally done at fixedfrequencies, the compensation factors may be pre-computed. Third,compensating the measured impedances using the coefficients determinedfrom the open/short/Load method. Fourth, mathematically removing thecable effect(s) on measured impedance so that only the sensing coilimpedance remains. Note that in a system that is trimmed for thismethod, the cables will be physically trimmed so that they match asclosely as possible the cable compensation values programmed into thesystem 10. Fifth, computing the function γ, which is the reactance ofthe coil at one frequency divided by the reactance of the same coil at adifferent frequency. Sixth, determining gap from the value γ, andseventh, iteratively repeating the previous six steps.

[0256] The advantage of the inductive ratio method according to theinstant invention is that many of the variables that affect theimpedance measurement are eliminated using this method. Some of thevariables eliminated are: the probe resistance, the probe inductance,the value of the known resistance in the detector, the voltage magnitudeand phase driven into the known resistor and the reference driving theanalog to digital converters.

[0257] As noted, this method determines gap in a way that is veryinsensitive to the series resistance of the coil. This is importantbecause most probe system failures cause a change in series resistance,but not in coil inductance. Errors like: loose connectors, temperaturevariations, and most significantly water in the probe all cause a changein resistance, but little or no change in reactance. The only singleended error sources left are the probe geometry and the referencedriving the digital to analog output.

[0258] Additionally, this method can be used as a way of detecting thegap of probe that is installed in the machine, detecting the gap of aprobe that may be contaminated with water thereby precluding theresistive term of impedance from being used and/or detecting the gap ofa probe wherein a far gap impedance can not be estimated to normalizethe measured impedance of the probe. Furthermore, this method can beused to provide redundant measurements of gap in the digital proximitysystem 10 thereby providing a cross check of actual system performance.

[0259] In operation, and referring to the drawings, the proximity probe12 is typically coupled proximate a target to be monitored, for example,a rotating shaft of a machine or an outer race of a rolling elementbearing for monitoring the gap therebetween. Therefore, the probe isstrategically coupled to the machine for sensing raw dynamic data thatis correlative to the spacing between the probe and the target of themachine being monitored to obtain a signature of the status of themachine.

[0260] The dynamic voltages V₁ and V₂ are continuously converted intoperiods or cycles of digital samples. The periods or cycles of digitalsamples are subsequently convolved into corresponding periods or cyclesof complex voltage numbers V_(1C) and V_(2C) which are used to determinedynamic impedance values of the probe 12. The dynamic impedance valuesare typically correlated to gap values correlative to the displacementmotion and position of the conductive target material being monitoredrelative to the probe. The impedance or gap values may be outputted toan analog output via a digital to analog converter 142. The analogoutput may be in the form of alarms, circuit breakers, etc. Thesedevices are set to trip when the analog output is outside a user setnominal operating range.

[0261] The impedance or gap values may be outputted to a host computer146 and/or to a processor 160 for further processing for the use ofmonitoring rotating or reciprocating machinery. In one form, andreferring to FIGS. 1 and 3, the digital proximity system 10 includes acommunications link 144, for example, a serial communications channel orinterface which is operatively coupled to the digital signal processormeans 110 for outputting signals to the host computer 146 and thus, toan end user. The serial communications channel 144 allows the digitalsamples or the convolved signals to be outputted from the digital signalprocessor means 110 to the remote computer 120 without sending the fulldynamic information of the original signals V₁ and V₂ thereby providingan important advantage of reducing the bandwidth of the communicationsignals. Additionally or alternatively, the impedance or gap values maybe outputted to the processor 160 where the values may be continuouslyaccumulated, processed and/or stored and, at any time, can betransmitted to the host computer 146 for further processing and/oroutput to an end user for the use of monitoring rotating orreciprocating machinery.

[0262] Furthermore, the digital signal processor means 110 or theprocessor 160 may perform signal reduction on the digitized impedance orgap values and then output that information to the remote computer 120via the serial communications link 118 and/or directly to the digital toanalog converter 116. For example, the digital signal processor means110 or the processor 150 may perform signal reduction in the from ofpeak to peak amplitude detection, DC gap detection, nX amplitude andphase detection and/or spectral content detection.

[0263] Moreover, memory means 120 can include an EEPROM tied to the DSPsuch that when the DSP first powers up it loads operating informationand parameters stored in EEPROM into its internal memory 112. The EEPROMcan be replaced with a dual port RAM (DPRAM) and as far as the DSP isconcerned it looks like an EEPROM. In one form, the processor means 160is coupled to the DPRAM and in turn, DPRAM is coupled to the DSP. Thus,when the DSP is held in recess the processor means 160 can loadprogramming and data into the DPRAM and once the DSP is released the DSPpulls everything out from the DPRAM and into internal memory 112.

[0264] The system 10 provides timely, meaningful and actionableinformation to end users. The behind the scenes activities that thesystem 10 may perform to verify its own condition and validate its datais a process which is not one task or idea, but a process by which thesystem 10 self-validates. The system 10 enables some level of additionalself-checking over existing systems. It is these aspects-of theaforementioned process, which are as follows:

[0265] 1. The system 10 can self identify target materials (or designedto work with all metals) thereby resulting in a system 10 which becannot mis-calibrated when put into operation.

[0266] 2. Multiple signal processing algorithms may be run at the sametime on the system 10. This allows cross-checking the different methodsdescribed above to verify proper system operation with the same eddycurrent probe. As an example, the inductive ratio method can be used tohelp tell if a probe is wet while it is still in the machine.

[0267] 3. The system 10 input bandwidth is sufficiently high to be ableto detect intermittent connections on the probe and extension cable. Forexample, an open or short will cause a sudden change in voltage at theanalog to digital converters. This change is faster than a rotor canmove to cause a change in analog to digital readings. Thus, by checkingthe slew rate of the signal we can check to see if it is faster than therotor can move. If it is to fast, the cause must be an electrical faultlike an intermittent connector. If the bandwidth was too slow, we couldnot differentiate the problems.

[0268] 4. The memory allows the system 10 to know if it has anintermittent on its input power wiring and the digital communicationschannel or link allows it to communicate its problems to an assetmanagement software system at the host. As an example, the system 10know that it reset due to power glitches three times in the last hourand digitally communicates that to an asset management system (Hostcomputer) which could check to see if the power had actually been turnedoff. If not, there is trouble with the wiring.

[0269] 5. The digital communications channel and the memory associatedwith the DSP and/or the CPU allows the system to generate it's ownmaintenance requests.

[0270] 6. The memory associated with the DSP and/or the CPU, and thedigital communications channel allows the system to store a completerecord of it's own checkout following installation. This allows thesystem to be able to communicate back up to the management softwarewhether or not it's been subject to a loop check, when that occurred andthe results of that test when it was run.

[0271] 7. The memory associated with the DSP and/or a CPU, and thedigital communications channel allows the system to be used with aportable checkout device that includes a bar code reader for recordingserial numbers on probes and extension cables. This allows theconfiguration software to upload which probe is tied to which extensioncable from the system 10. Standard labels could also be providedindicating bearing number and X,Y, spare X, spare Y to link these intothe system 10. This helps eliminate translation errors caused by havinga user write the data down and then keypunching them into the system 10.

[0272] 8. Including the complete signal chain in the system 10 software(including serial numbers) allows remote access for a product servicegroup to look for configuration errors or trace repairs that may need tobe done. It also allows spares to be ordered without having to havesomeone go and look at the installation.

[0273] 9. The barcode technology can work the other way when the systems10 is disconnected and re-assembled. The portable device can request theprobe/extension cable serial numbers from the system 10 and then make atone or a beep representing good or bad as the technician scans barcodeslooking for the right one to tie into the system.

[0274] 10. The system 10 can include a signal processing algorithm thatis essentially immune to gap and very sensitive to material condition(sort of an electrical runout measurement system). This can be used tocreate a waveform representing the material condition of the shaft. Thispattern may be compared between X-Y pairs to help verify that the probeorientation and direction of rotation are all configured correctly.

[0275] 11. The system 10 can stop driving the probe and extension cable,but still measure the voltage developed across it due to ground loops orRFI. This can be done during system assembly as a check.

[0276] 12. The system 10 can measure the wideband RMS voltage andcompare that to the one frequency that the system 10 is measuring at tosee if noise is being injected into the signal for some reason. Note,the system 10 may not be getting bad readings because of the narrowbandwidth and the system 10 is still able to detect that the signalnoise is there. This can be correlated to the “bump in the night” datathat may cause some kind of glitch. This is very similar to a NOT 1Xmeasurement made, however the system can discern exactly how muchsynchronous signal is being driven through the system so any NOT 1X willbe due to harmonic distortion (which should not change unless a hardwareproblem occurs) and outside noise. If necessary, one could computespectra of the signal and separate out harmonic distortion (an internalhardware problem) with internally or externally generated noise. It'salso possible to compute the spectra using different samplingfrequencies and figure out the exact frequency that's causing trouble(assuming it's not wideband noise). This is because one will be able toidentify where the foldover frequencies occur and can identify thealiased frequencies.

[0277] 13. The 1X signal may also be used to help verify probeorientation and direction of rotation.

[0278] 14. An internal timestamped event list may be maintained in thesystem to document when changes were made to it's configuration. Thiscan be used to help verify that there were no NOT OK times from the timeof some recent event back to its last verification cycle.

[0279] Furthermore, the system 10 also provides a solution to a need formore systems to be used as references (working standards) duringmanufacturing process of analog systems. The stability of the system 10increases at a result of at least the following three reasons: One, thesystem 10 design is inherently more stable because it depends on theratiometric measurements used in the Analog to Digital (A-D) converters,rather than on the bias through a PN junction operating on, for example,a 1 MHz signal. Two, the tank inductor in the analog system has beeneliminated and replaced with a mathematical equation. The tank inductoris the most sensitive component and has a tendency to “walk” over time.“Walk” refers to a ferrite core inductor's tendency to experience shortand long term drift in its impedance value. It is not known what causesit, but it is known that it's there. Three, it is possible to performopen/short/load calibration on every working standard system 10 at thebeginning of the day or work order to re-zero it's response.

[0280] Moreover, having thus described the invention, it should beapparent that numerous structural modifications and adaptations may beresorted to without departing from the scope and fair meaning of theinstant invention as set forth hereinabove and as described hereinbelowby the claims.

I Claim:
 1. A device for digitally measuring electrical impedance,comprising in combination: a network including a first electricalcomponent and a second electrical component serially connected; a signalgenerating means operatively coupled to said network for driving acurrent through said serially connected components; means for sampling afirst voltage impressed across said network and a second voltageimpressed across said second component into digitized voltages; meansfor convolving each said digitized voltage with a digital waveform forforming a first complex number and a second complex number correlativeto said first voltage impressed across said network and said secondvoltage impressed across said second component respectively; means fordetermining a ratio of said second complex number to a differencebetween said first and said second complex number, and means forcalculating an electrical impedance of said second component bymultiplying said ratio by a value of said first component wherein saidelectrical impedance of said second component is digitally measured. 2.The device of claim 1 wherein said convolving means includes means,operatively coupled to said sampling means, for digitally multiplyingeach of said digitized voltages with a digitized sine wave and adigitized cosine wave.
 3. The device of claim 2 wherein said digitizedsine wave and said digitized cosine wave are pulled from a memory meansoperatively coupled to said convolving means.
 4. The device of claim 3wherein said convolving means further includes means for transformingthe results of multiplying each of said digitized voltages with saiddigitized sine wave and said digitized cosine wave into orthogonal DCcomponents which define a real component magnitude and an imaginarycomponent magnitude for each of said first complex number and saidsecond complex number correlative to said first voltage impressed acrosssaid network and said second voltage impressed across said secondcomponent.
 5. The device of claim 4 wherein said determining meansincludes a digital signal processor operatively coupled to saidconvolving means for determining said ratio of said second complexnumber to said difference between said first and said second complexnumber, and wherein said calculating means includes said processor forcalculating said unknown electrical impedance of said second componentby multiplying said ratio by said know value of said first component. 6.The device of claim 5 wherein said signal generating means includes atleast one frequency/phase programmable signal generator operativelycoupled to said digital signal processor and said network for receivingdigital commands from said digital signal processor for generating afrequency/phase programmable signal for driving said current throughsaid serially connected components at a digitally programmedfrequency/phase defined by said digital signal processor.
 7. The deviceof claim 6 further including a timing control means for triggering saidsampling means to sample said first voltage and said second voltage at arate which maintains a substantially constant phase relationship betweensaid programmable signal and the sampled voltages.
 8. The device ofclaim 1 wherein said first electrical component includes a resistancemeans having a known resistance value.
 9. The device of claim 8 whereinsaid second electrical component includes a proximity probe comprising asensing coil and a probe cable operatively coupled to said sensing coil,said proximity probe having an unknown impedance value and locatedproximate a conductive target material to be monitored for position. 10.The device of claim 9 wherein said means for calculating said electricalimpedance of said second component by multiplying said ratio by saidvalue of said first component includes calculating the electricalimpedance of the proximity probe by multiplying said ratio by said knowresistance value wherein said unknown electrical impedance of saidproximity probe is digitally measured.
 11. A device of claim 10 furtherincluding means for correlating the digitally measured impedance valueof the proximity probe to a value defining a gap between said proximityprobe and said metallic target being monitored.
 12. A method fordigitally measuring electrical impedance, the steps including: forming anetwork including providing a first electrical component and a secondelectrical component serially connected; driving the network with adynamic signal for impressing a voltage across the network and eachcomponent; digitizing the voltage across the network and the voltageacross the second electrical component; convolving each of the digitizedvoltages with a digital waveform for forming a first complex number anda second complex number correlative to the voltages across the networkand across the second electrical component respectively; determining aratio of the second complex number to a difference between the firstcomplex number and the second complex number; calculating an electricalimpedance of the second electrical component by multiplying the ratio bya know digitized value of the first electrical component wherein saidelectrical impedance of the second component is digitally measured. 13.The method of claim 12 wherein the step of convolving each of thedigitized voltages with the digital waveform includes the step ofmultiplying each of the digitized voltages with a digital sine waveformand a digital cosine waveform and then accumulating and averaging eachmultiply for forming the first complex number and the second complexnumber including the real and the imaginary magnitudes correlative ofthe voltage impressed across the network and across the secondelectrical component respectively.
 14. The method of claim 13 whereinthe step of convolving each of the digitized voltages with the digitalwaveform includes the step of selecting from a memory means the digitalsine waveform and the digital cosine waveform.
 15. The method of claim14 wherein the step of driving the network with the dynamic signal forimpressing the voltage across the network and each component furtherincludes the step of digitally programming a frequency of the dynamicwaveform.
 16. The method of claim 15 wherein the step of digitizing thevoltage across the network and across the second electrical componentincludes the step of sampling the voltage across the network and acrossthe second electrical component at a rate which maintains asubstantially constant phase relationship between the dynamic signal andthe sampled voltages.
 17. The method of claim 12 wherein the step offorming the network including providing the first electrical componentincludes the step of providing the first electrical component in theform of a resistance means having a known resistance value.
 18. Themethod of claim 17 wherein the step of forming the network includingproviding the second electrical component includes the step of providingthe second electrical component in the form of a proximity probeincluding a sensing coil and a probe cable operatively coupled to saidsensing coil, the proximity probe having an unknown impedance.
 19. Themethod of claim 18 further including the step of locating the proximityprobe proximate a metallic target to be monitored.
 20. The method ofclaim 19 wherein the step of calculating the electrical impedance of thesecond electrical component by multiplying the ratio by the know valueof the first electrical component includes calculating the electricalimpedance of the proximity probe by multiplying the ratio by the knowresistance value wherein the unknown electrical impedance of theproximity probe is digitally measured.
 21. A method of claim 20 furtherincluding the step of correlating the digitally measured impedance valueof the proximity probe to a value defining a gap between the proximityprobe and the metallic target being monitored.
 22. An apparatus fordetermining a gap between a proximity probe and a conductive targetmaterial, said apparatus comprising in combination: a network includinga first electrical component and a proximity probe serially connected; asignal generating means operatively coupled to said network for drivinga current through said serial connection wherein a first analog voltageis impressed across said network and a second analog voltage isimpressed across said proximity probe; means for sampling and digitizingsaid first analog voltage impressed across said network and said secondanalog voltage impressed across said proximity probe into digitizedvoltages; means for convolving each said digitized voltage with adigital waveform for forming a first complex number and a second complexnumber correlative to said first analog voltage impressed across saidnetwork and said second analog voltage impressed across said proximityprobe respectively; means for determining a voltage ratio of said secondcomplex number to a difference between said first complex number andsaid second complex number; means for processing said voltage ratio intoa gap value correlative to a gap between said proximity probe and aconductive target material.
 23. The apparatus of claim 22 wherein saidprocessing means further includes means for calculating a complexelectrical impedance value of said proximity probe from said voltageratio and processing said impedance value into said gap valuecorrelative to said gap between said proximity probe and said conductivetarget material.
 24. The apparatus of claim 23 further including meansfor outputting a signal as a function of said gap value which iscorrelative to said gap between said proximity probe and said conductivetarget material.
 25. The apparatus of claim 22 wherein said processingmeans further includes means for calculating a complex electricalimpedance value of said proximity probe from said voltage ratio andnormalizing said complex electrical impedance value into said gap valuecorrelative to said gap between said proximity probe and said conductivetarget material.
 26. The apparatus of claim 25 further including meansfor outputting a signal as a function of said gap value which iscorrelative to said gap between said proximity probe and said conductivetarget material.
 27. An apparatus for determining a gap between aproximity probe and a conductive target material, said apparatuscomprising in combination: a network including an extension cableinterposed between and serially connected to a first electricalcomponent and a proximity probe; a signal generating means operativelycoupled to said network for driving a current through said serialconnection wherein a first analog voltage is impressed across saidnetwork and a second analog voltage is impressed across said serialconnection of said extension cable and said proximity probe; means forsampling and digitizing said first analog voltage impressed across saidnetwork and said second analog voltage impressed across said serialconnection of said extension cable and said proximity probe intodigitized voltages; means for convolving each said digitized voltagewith a digital waveform for forming a first complex number and a secondcomplex number correlative to said first analog voltage impressed acrosssaid network and said second analog voltage impressed across said serialconnection of said extension cable and said proximity proberespectively; means for determining a voltage ratio of said secondcomplex number to a difference between said first complex number andsaid second complex number; means for processing said voltage ratio intoa gap value correlative to a gap between said proximity probe and aconductive target material.
 28. The apparatus of claim 27 wherein saidprocessing means further includes means for calculating a complexelectrical impedance value of said proximity probe from said voltageratio by digitally compensating for an impedance of said extension cableand processing said complex electrical impedance value into said gapvalue correlative to said gap between said proximity probe and saidconductive target material.
 29. The apparatus of claim 28 furtherincluding means for outputting a signal as a function of said gap valuewhich is correlative to said gap between said proximity probe and saidconductive target material.
 30. The apparatus of claim 27 wherein saidprocessing means further includes means for calculating a complexelectrical impedance value of said proximity probe from said voltageratio by digitally compensating for an impedance of said extension cableand normalizing said complex electrical impedance value into said gapvalue correlative to said gap between said proximity probe and saidconductive target material.
 31. The apparatus of claim 30 furtherincluding means for outputting a signal as a function of said gap valuewhich is correlative to said gap between said proximity probe and saidconductive target material.
 32. The apparatus of claim 27 furtherincluding means for storing proximity probe and extension cable serialsnumbers for identifying said proximity probe and said extension cable insaid network.
 33. The apparatus of claim 32 further including a portablecheck out device operatively coupled to said storing means and includinga bar code reader for recording into said storing means serial numberson probes and extension cables for identifying which probe is coupled towhich extension cable.
 34. The apparatus of claim 33 wherein said serialnumbers of probes and extension cables coded to indicate machinelocation and orientation.
 35. The apparatus of claim 32 furtherincluding a portable check out device operatively coupled to saidstoring means for receiving stored probe and extension cable serialsnumbers and including a bar code reader for scanning probes andextension cables for identifying a match to the stored probe andextension cable serials numbers.
 36. The apparatus of claim 27 furtherincluding means for determining intermittent connections on said probeand said extension cable by monitoring rates of change of said analogvoltage signals sampled by said sampling means and determining if saidrates of change are greater than a rate that said conductive targetmaterial can move.
 37. The apparatus of claim 27 further including meansfor determining an intermittent on input power wiring to the apparatusby determining if power to said apparatus has caused said apparatus toreset over a certain period of time and means for communicating thisinformation to a host computer operatively coupled to said processingmeans.
 38. The apparatus of claim 27 further including a memory meansoperatively coupled to said processing means which in turn isoperatively coupled to a host computer wherein a maintenance requeststored in said memory means is communicated to said host computer fromsaid apparatus.
 39. The apparatus of claim 27 further including a memorymeans operatively coupled to said processing means for storing checkoutinformation following installation in order to communicate to a hostcomputer operatively coupled to said processing means wherein the hostcomputer can discern whether said apparatus has been subject to a loopcheck, when the check occurred and results of the check.
 40. Theapparatus of claim 27 further including a memory means operativelycoupled to said processing means which in turn is operatively coupled toa host computer for allowing remote access to the apparatus fordetermining configuration errors and needed repairs wherein requiredreplacement components can be obtained without having to go to thelocation of said apparatus.
 41. The apparatus of claim 40 wherein atimestamped event list is maintained in said memory means to documentconfiguration changes.
 42. The apparatus of claim 27 wherein saidsampling and digitizing means samples and digitizes an analog voltageimpressed across said serial connection of said extension cable and saidproximity probe into digitized voltages without being driven by saidalternating current and processing said voltages by said processingmeans for determining signals correlative to ground loops or radiofrequency interference.
 43. The apparatus of claim 27 wherein saidprocessing means measures a wideband RMS voltage from said digitizedvoltages and compares that to a frequency of the signal driving theprobe to determine if noise is being injected into the signal drivingthe probe.
 44. An apparatus for determining a dynamic gaps between aproximity probe and a conductive target material, said apparatuscomprising in combination: means for establishing dynamic voltagesignals correlative to dynamic gaps between a proximity probe and aconductive target material; sampling means for digitizing saidestablished dynamic voltage signals into digital voltage signals; adigital multiplier for multiplying each said digital voltage signal by adigital sine signal and a digital cosine signal; means for accumulatingvalues of each multiply in a memory, and means for processing eachmultiply for obtaining complex voltage representations correlative todynamic gaps between said proximity probe and a conductive targetmaterial.
 45. The apparatus of claim 44 wherein said means forestablishing dynamic voltage signals correlative to dynamic gaps betweensaid proximity probe and said conductive target material includes anetwork including a first electrical component and said proximity probeserially connected, and a signal generating means operatively coupled tosaid network for driving a current through said serial connectionwherein a first analog voltage is impressed across said network and asecond analog voltage is impressed across said proximity probe forestablishing said dynamic voltage signals.
 46. The apparatus of claim 45wherein said sampling means for digitizing said established dynamicvoltage signals into digital voltage signals includes means for samplingand digitizing said first analog voltage impressed across said networkand said second analog voltage impressed across said proximity probeinto first and second digitized voltages respectively.
 47. The apparatusof claim 46 wherein said digital multiplier multiplies each of saidfirst and second digitized voltages by said digital sine signal and saiddigital cosine signal for forming first complex number and secondcomplex number pairs.
 48. The apparatus of claim 47 wherein said meansfor processing each multiply for obtaining said complex voltagerepresentations correlative to said dynamic gaps between said proximityprobe and said conductive target material includes means for determiningvoltage ratios of each said complex number pair by determining a ratioof said second complex number to a difference between said first complexnumber and said second complex number for each said pair and processingwith said processing means said voltage ratios into values correlativeto said dynamic gaps between said proximity probe and said conductivetarget material.
 49. A method for measuring a gap between a proximityprobe and a conductive target material, said method including the stepsof: providing a network of components including a first electricalcomponent and a proximity probe component serially connected; driving adynamic current through the serially connected electrical components forimpressing a first analog voltage across the network and a second analogvoltage cross the proximity probe component; sampling and digitizing thefirst analog voltage impressed across said serially connected resistanceand probe components to obtain a first digitized voltage value; samplingand digitizing a second analog voltage impressed across the probecomponent to obtain a second digitized voltage value; digitallyconvolving said first digitized voltage and said second digitizedvoltage into a first complex number and a second complex numberrespectively; calculating a voltage ratio of said second complex numberto a difference between said first complex number and said secondcomplex number; processing said voltage ratio into a gap valuecorrelative to a gap between said proximity probe and a conductivetarget material.
 50. The method of claim 49 further including the stepof outputting a signal as a function of said gap value which iscorrelative to said gap between said proximity probe and said conductivetarget material.
 51. The method of claim 49 wherein said processing stepfurther includes the step of calculating a complex electrical impedancevalue of said proximity probe from said voltage ratio and processingsaid impedance value into said gap value correlative to said gap betweensaid proximity probe and said conductive target material.
 52. The methodof claim 51 further including the step of outputting a signal as afunction of said gap value which is correlative to said gap between saidproximity probe and said conductive target material.
 53. The apparatusof claim 49 wherein said processing step further includes the steps ofcalculating a complex electrical impedance value of said proximity probefrom said voltage ratio, normalizing said complex electrical impedancevalue into a normalized value and processing said normalized value intosaid gap value correlative to said gap between said proximity probe andsaid conductive target material.
 54. The apparatus of claim 53 furtherincluding means for outputting a signal as a function of said gap valuewhich is correlative to said gap between said proximity probe and saidconductive target material.
 55. A method for measuring a gap between aproximity probe and a conductive target material, said method includingthe steps of: providing a network of components including a firstelectrical component and a proximity probe component serially connected;driving a dynamic current through the serially connected electricalcomponents including the resistance component and the proximity probecomponent for impressing a first analog voltage across the network and asecond analog voltage cross the proximity probe component; sampling anddigitizing the first analog voltage impressed across said seriallyconnected resistance and probe components to obtain a first digitizedvoltage value; sampling and digitizing a second voltage impressed acrossthe probe component to obtain a second digitized voltage value;digitally convolving said first digitized voltage and said seconddigitized voltage into a first complex number and a second complexnumber respectively; calculating a voltage ratio of said second complexnumber to a difference between said first complex number and said secondcomplex number; multiplying the voltage ratio by a value of the firstelectrical component for determining an impedance of the proximityprobe; correlating the determined impedance of the proximity probe to agap between the proximity probe and a conductive target material. 56.The method of claim 55 further including the step of storing digitalvalues in a memory means for defining a plurality different gap values.57. The method of claim 56 wherein the correlating step further includesthe step of correlating the determined impedance to at least one storeddigital value for determining the gap between the proximity probe andthe conductive target material.
 58. A method for measuring a gap betweena proximity probe and a conductive target material, said methodincluding the steps of: providing a network of components including afirst electrical component, an extension cable component and a proximityprobe component respectively serially connected, and locating theproximity probe adjacent a conductive target material; driving a dynamiccurrent through the serially connected electrical components forimpressing a first analog voltage across the network and a second analogvoltage across the serial connection of the extension cable componentand the proximity probe component; sampling and digitizing the firstanalog voltage impressed across said network to obtain a first digitizedvoltage value; sampling and digitizing a second analog voltage impressedacross the serial connection of the extension cable component and theproximity probe component to obtain a second digitized voltage value;digitally convolving the first digitized voltage value and the seconddigitized voltage value into a first complex number and a second complexnumber respectively; calculating a voltage ratio of the second complexnumber to a difference between the first complex number and the secondcomplex number; processing the voltage ratio into a gap valuecorrelative to a gap between the proximity probe and the conductivetarget material.
 59. The method of claim 58 wherein said processing stepfurther includes the step of calculating a complex electrical impedancevalue of the proximity probe from the voltage ratio by digitallycompensating for an impedance of the extension cable and processing thecomplex electrical impedance value into said gap value correlative tosaid gap between said proximity probe and said conductive targetmaterial.
 60. The method of claim 59 further including the step ofoutputting a signal as a function of said gap value which is correlativeto said gap between said proximity probe and said conductive targetmaterial.
 61. The method of claim 58 wherein said processing stepfurther includes the steps of calculating a complex electrical impedancevalue of the proximity probe from the voltage ratio by digitallycompensating for an impedance of said extension cable and normalizingthe complex electrical impedance value into said gap value correlativeto said gap between said proximity probe and said conductive targetmaterial.
 62. The apparatus of claim 61 further including means foroutputting a signal as a function of said gap value which is correlativeto said gap between said proximity probe and said conductive targetmaterial.
 63. The method of claim 58 wherein said processing stepfurther includes the step of determining a complex electrical impedanceof the serial connection of the extension cable and proximity probe bymultiplying the voltage ratio by a value of the electrical component,the determined value defining a measured value of the electricalimpedance of the serial connection of the extension cable and proximityprobe.
 64. The method of claim 63 further including the step ofperforming open, short and load measurements on the extension cable fordetermining compensation parameters and storing the parameters in amemory means, the compensation parameters including a measured openimpedance of the extension cable, a measured short impedance of theextension cable, a measured impedance of a load coupled to the extensioncable in place of the proximity probe, the load having a known impedancevalue.
 65. The method of claim 64 further including the step ofdetermining a complex electrical impedance of the probe by utilizing thecompensation parameters and by utilizing the formula:$Z_{probe} = \frac{{Z_{s\quad t\quad d}\left( {Z_{O} - Z_{S\quad M}} \right)}\left( {Z_{X\quad M} - Z_{S}} \right)}{\left( {Z_{S\quad M} - Z_{S}} \right)\left( {Z_{O} - Z_{X\quad M}} \right)}$

where: Z_(O) is the measured open impedance of the extension cable,Z_(S) is the measured short impedance of the extension cable, Z_(SM) isthe measured impedance of the load coupled to the extension cable inplace of the proximity probe, Z_(std) is the known value of the loadimpedance, Z_(XM) is the measured impedance of the probe coupled to theextension cable, Z_(probe) is the compensated complex electricalimpedance of the proximity probe.
 66. The method of claim 65 whereinsaid processing step further includes the step of processing the complexelectrical impedance of the proximity probe into said gap valuecorrelative to said gap between said proximity probe and said conductivetarget material.
 67. The method of claim 66 further including the stepof outputting a signal as a function of said gap value which iscorrelative to said gap between said proximity probe and said conductivetarget material.
 68. The method of claim 65 wherein said processing stepfurther includes the steps of normalizing the complex electricalimpedance value of the probe into a normalized impedance value andprocessing the normalized impedance value into said gap valuecorrelative to said gap between said proximity probe and said conductivetarget material.
 69. The apparatus of claim 68 further including meansfor outputting a signal as a function of said gap value which iscorrelative to said gap between said proximity probe and said conductivetarget material.
 70. The method of claim 58 further including the stepof storing electrical parameters for a plurality of different proximityprobes and extension cables and retrieving the electrical parameters ofthe proximity probe that is disposed adjacent the conductive targetmaterial and the electrical parameters of the extension cable coupled tothat probe for use in the step of processing the voltage ratio into agap value correlative to a gap between the proximity probe and theconductive target material.
 71. The method of claim 58 further includingthe step of storing electrical parameters for a plurality of differenttarget materials and retrieving the electrical parameters of the targetmaterial in which the proximity probe is disposed adjacent to and usingthe retrieved parameters in the step of processing the voltage ratiointo a gap value correlative to a gap between the proximity probe andthe conductive target material.
 72. A method for measuring a position ofa conductive target material, the steps including: sampling anddigitizing a first voltage impressed across a serial connection of aresistance means and a proximity probe located adjacent a conductivetarget material to obtain a first digitized voltage; sampling anddigitizing a second voltage impressed only across the probe to obtain asecond digitized voltage, transforming the two digitized voltages intocomplex voltage numbers; calculating an electrical impedance of theproximity probe by using both complex voltage numbers; correlating thecalculated electrical impedance to a gap between the proximity probe andthe conductive target material.
 73. The method of claim 72 furtherincluding the steps of normalizing the calculated electrical impedanceof the proximity probe and comparing the normalized impedance to aplurality of previously compiled representations of normalized impedancecurves for different conductive target materials for identifying theconductive target material prior to correlating the calculatedelectrical impedance to the gap between the proximity probe and theconductive target material.
 74. The method of claim 72 wherein thecorrelating step further includes the steps of normalizing thecalculated electrical impedance of the proximity probe and comparing thenormalized impedance to a previously compiled representation of aplurality of gap locus lines, each defining a single gap value fordifferent conductive target materials, for identifying a gap line foruse in correlating the calculated electrical impedance to a gap betweenthe proximity probe and the conductive target material.
 75. The methodof claim 70 further including the step of storing electrical parametersfor a plurality of different proximity probes and retrieving theelectrical parameters of the proximity probe that is disposed adjacentthe conductive target material for use in the step of calculating theelectrical impedance of the proximity probe.
 76. The method of claim 70further including the step of storing electrical parameters for aplurality of different target materials and retrieving the electricalparameters of the target material in which the proximity probe isdisposed adjacent and using the retrieved parameters in the step ofcalculating the electrical impedance of the proximity probe.
 77. Amethod for measuring a gap between a proximity probe and a conductivetarget material, the steps including: sampling and digitizing a firstvoltage impressed across a serial connection of a first electricalcomponent and a proximity probe located adjacent a conductive targetmaterial to obtain a first digitized voltage; sampling and digitizing asecond voltage impressed across the probe to obtain a second digitizedvoltage, transforming the two digitized voltages into complex voltagenumbers; determining an electrical impedance of the proximity probe byusing both complex voltage numbers; normalizing the electrical impedanceof the proximity probe; correlating the normalized electrical impedanceof the proximity probe to a gap between the proximity probe and theconductive target material.
 78. The method of claim 77 wherein thecorrelating step further includes the steps of comparing the normalizedelectrical impedance to a plurality of previously compiledrepresentations of normalized impedance curves for different conductivetarget materials for identifying the conductive target material prior tocorrelating the normalized electrical impedance to the gap between theproximity probe and the conductive target material.
 79. The method ofclaim 77 wherein the correlating step further includes the step ofcomparing the normalized impedance to a previously compiledrepresentation of a plurality of gap locus lines, each defining a singlegap value for different conductive materials, for identifying a gap linefor use in determining the gap between the proximity probe and theconductive target material.
 80. A method for measuring a gap between aproximity probe and a conductive target material, the steps including:sampling and digitizing a first voltage impressed across a serialconnection of a first electrical component, an extension cable and aproximity probe located adjacent a conductive target material to obtaina first digitized voltage; sampling and digitizing a second voltageimpressed across the probe to obtain a second digitized voltage,transforming the two digitized voltages into complex voltage numbers;determining an electrical impedance of the proximity probe by using bothcomplex voltage numbers and compensating for the extension cable;normalizing the electrical impedance of the proximity probe; correlatingthe normalized electrical impedance of the proximity probe to a gapbetween the proximity probe and the conductive target material.
 81. Themethod of claim 80 wherein the correlating step further includes thesteps of comparing the normalized electrical impedance to a plurality ofpreviously compiled representations of normalized impedance curves fordifferent conductive target materials for identifying the conductivetarget material prior to correlating the normalized electrical impedanceto the gap between the proximity probe and the conductive targetmaterial.
 82. The method of claim 81 wherein the correlating stepfurther includes the step of comparing the normalized impedance to apreviously compiled representation of a plurality of gap locus lines,each defining a single gap value for different conductive materials, foridentifying a gap line for use in determining the gap between theproximity probe and the conductive target material.
 83. A method formeasuring a gap between a proximity probe and a conductive targetmaterial, said method including the steps of: digitally measuring anelectrical impedance of a proximity probe located adjacent a conductivetarget material; combining a predetermined digitized impedance with thedigitally measured impedance of the proximity probe; correlating thecombined impedance to a gap interposed between the proximity probe andthe conductive target material being monitored.
 84. The method of claim83 wherein the step of combining the predetermined digitized impedancewith the measured impedance of the proximity probe includes combining anequivalent digitized impedance of an analog circuit with the digitallymeasured impedance of the proximity probe.
 85. The method of claim 83wherein the step of correlating the combined impedance to the gapinterposed between the proximity probe and the conductive targetmaterial being monitored includes the step of correlating both thecombined impedance and a measured frequency of a signal driving theproximity probe to the gap interposed between the probe and theconductive target material being monitored.
 86. The method of claim 85further including the step of adjusting the frequency of the signaldriving the proximity probe as a function of the combined impedance andthe measured frequency for correcting any anomalous phase error andsubsequently driving the proximity probe at the adjusted frequency. 87.The method of claim 83 further including the step of compensating thedigitally measured impedance of the proximity probe by usingcompensation coefficients for eliminating probe cable effects includedin the proximity probe.
 88. The method of claim 87 wherein the step ofcompensating the digitally measured impedance of the proximity probe byusing compensation coefficients includes the steps of performing anopen, a short and a load measurement on the probe cable for determiningthe compensation coefficients.
 89. A method for measuring a gap betweena proximity probe and a conductive target material, said methodincluding the steps of: digitally measuring an electrical impedance ofan a proximity probe and an extension cable connected thereto, theproximity probe is located adjacent a conductive target material;combining a predetermined digitized impedance with the digitallymeasured impedance; correlating the combined impedance to a gapinterposed between the proximity probe and the conductive targetmaterial being monitored.
 90. The method of claim 89 wherein combiningthe predetermined digitized impedance with the digitally measuredimpedance further includes the step of compensating the digitalimpedance of the proximity probe and extension cable by usingcompensation coefficients for eliminating the extension cable effects onthe digitally measured impedance of the proximity probe and extensioncable for forming the combined impedance.
 91. The method of claim 90wherein the step of compensating the digitally measured impedance of theproximity probe and extension cable by using compensation coefficientsincludes the steps of performing an open, a short and a load measurementon the extension cable for determining compensation coefficients andstoring the coefficients in a memory means, the compensationcoefficients including a measured open impedance of the extension cable,a measured short impedance of the extension cable and a measuredimpedance of a load coupled to the extension cable in place of theproximity probe, the load having a known impedance value.
 92. The methodof claim 91 wherein the step of compensating the digitally measuredimpedance of the proximity probe and extension cable further includesthe step of determining a compensated digitally measured impedance byutilizing the compensation coefficients and by utilizing the formula:$Z_{compensated} = \frac{{Z_{std}\left( {Z_{O} - Z_{SM}} \right)}\left( {Z_{XM} - Z_{S}} \right)}{\left( {Z_{SM} - Z_{S}} \right)\left( {Z_{O} - Z_{XM}} \right)}$

where: Z_(OM) is the measured open impedance of the extension cable,Z_(S) is the measured short impedance of the extension cable, Z_(SM) isthe measured impedance of the load coupled to the extension cable inplace of the proximity probe, Z_(std) is the known value of the loadimpedance, Z_(XM) is the measured value of the electrical impedance ofthe serial connection of the extension cable and proximity probe,Z_(compensated) is the compensated digitally measured impedance.
 93. Themethod of claim 92 wherein the step of combining the predetermineddigitized impedance with the digitally measured impedance includescombining the predetermined digitized impedance with the compensateddigitally measured impedance for forming the combined impedance.
 94. Themethod of claim 93 wherein the step of correlating the combinedimpedance to the gap interposed between the proximity probe and theconductive target material being monitored further includes the steps ofnormalizing the combined impedance into a normalized impedance value andprocessing the normalized impedance value into a gap value correlativeto the gap between said proximity probe and the conductive targetmaterial.
 96. The method of claim 94 wherein the correlating stepfurther includes the steps of comparing the normalized impedance valueto a plurality of previously compiled representations of normalizedimpedance curves for different conductive target materials foridentifying the conductive target material prior to processing thenormalized impedance value into a gap value correlative to the gapbetween said proximity probe and the conductive target material.
 97. Themethod of claim 94 wherein the correlating step further includes thestep of comparing the normalized impedance value to a previouslycompiled representation of a plurality of gap locus lines, each defininga single gap value for different conductive materials, for identifying agap line for use in the step of processing the normalized impedancevalue into a gap value correlative to the gap between the proximityprobe and the conductive target material.
 98. The method of claim 89wherein the step of combining the predetermined digitized impedance withthe digitally measured impedance includes combining an equivalentdigitized impedance of an analog circuit with the digitally measuredimpedance.
 99. A method for measuring a gap between a proximity probeand a conductive target material, said method including the steps of:measuring an impedance of a proximity probe located proximate aconductive target material and an extension cable operatively coupled tothe proximity probe; compensating the measured impedance by usingcompensation coefficients stored in a memory means; combining apredetermined impedance with the compensated measured impedance forforming a combination impedance; determining a gap between the proximityprobe and the conductive target material as a function of thecombination impedance; iteratively repeating the measuring,compensating, combining and determining steps to substantiallycontinuously monitor the gap between the probe and the target as afunction of the combination impedance.
 100. The method of claim 99wherein the step of combining the predetermined impedance with themeasured impedance includes combining an equivalent impedance of ananalog circuit with the digitally compensated impedance.
 101. A methodfor measuring a gap between a proximity probe and a conductive targetmaterial, the steps including: providing a database of normalizedimpedance curve representations for different conductive targetmaterials; measuring an impedance of a proximity probe located proximatea conductive target material being identified; normalizing the measuredprobe impedance; utilizing the normalized probe impedance and thedatabase of normalized impedance curve representations for identifyingthe conductive target material; determining a gap value between theproximity probe and the conductive target material from the normalizedprobe impedance and the identified target material.
 102. The method ofclaim 101 wherein the step of providing a database of normalizedimpedance curve representations for different conductive targetmaterials includes providing at least one lookup table storing values ofeach normalized impedance curve for each different target material. 103.The method of claim 102 wherein the step of providing a database ofnormalized impedance curve representations for different conductivetarget materials includes providing at least one equation in thedatabase to represent normalized impedance curves for different targetmaterials.
 104. A method for measuring a gap between a proximity probeand a conductive target material, the steps including: providing arepresentation of a defined series of gap locus each representative ofthe same gap for different target materials; measuring an impedance of aproximity probe located proximate a conductive target material;normalizing the measured probe impedance; determining a gap valuebetween the proximity probe and the conductive target material from thenormalized probe impedance and the representation of the defined seriesof gap locus wherein the gap value is substantially correct for anyconductive target material adjacent the proximity probe therebyproviding a material insensitive method for measuring gap values betweenthe proximity probe and different conductive target materials.
 105. Themethod of claim 104 wherein the step of providing the representation ofthe defined series of gap locus that represent the same gap fordifferent target materials includes using a lookup table to representthe series of gap locus.
 106. The method of claim 104 wherein the stepof providing the representation of the defined series of gap locus thatrepresent the same gap for different target materials includes using atleast one equation to represent the series of gap locus.
 107. The methodof claim 104 wherein the determining step further includes comparing thenormalized probe impedance with the representation of the defined seriesof gap locus in the database for determining a gap locus which definesthe gap value between the proximity probe and the conductive targetmaterial wherein the gap value is substantially correct for anyconductive target material adjacent the proximity probe therebyproviding the material insensitive method for measuring gap valuesbetween the proximity probe and different conductive target materials.108. A method for measuring a gap between a proximity probe and aconductive target material, the steps including: providing arepresentation of a defined series of gap locus each representative ofthe same gap for different target materials; measuring an impedance of aproximity probe located proximate a conductive target material, theproximity probe including a probe cable; compensating an impedancecontribution of the probe cable from the measured probe impedance todefine a measured coil impedance; normalizing the measured coilimpedance; determining a gap value between the proximity probe and theconductive target material from the normalized coil impedance and therepresentation of the defined series of gap locus wherein the gap valueis substantially correct for any conductive target material adjacent theproximity probe thereby providing a material insensitive method formeasuring gap values between the proximity probe and differentconductive target materials.
 109. The method of claim 108 wherein thedetermining step further includes comparing the normalized coilimpedance with the representation of the defined series of gap locus inthe database for determining a gap locus that corresponds to thenormalized impedance value of the coil for defining the gap valuebetween the proximity probe and the conductive target material whereinthe gap value is substantially correct for different conductive targetmaterials adjacent the proximity probe thereby providing the materialinsensitive method for measuring gap values between the proximity probeand different conductive target materials.
 110. A method for measuring agap between a proximity probe and a conductive target material, thesteps including: measuring a proximity probe impedance at a firstfrequency and a second different frequency, the proximity probeincluding an integral sensing coil; determining an impedance of thesensing coil from the measured proximity probe impedance at the firstfrequency and the second different frequency; dividing a reactance ofthe impedance of the sensing coil at the first frequency by thereactance of the impedance of the sensing coil at the second differentfrequency for defining an inductive ratio; correlating the inductiveratio to a value representative to a gap between the proximity probe andthe conductive target material.
 111. A method for measuring a gapbetween a proximity probe and a conductive target material, the stepsincluding: sampling and digitizing a first voltage impressed across aserial connection of a resistance means and a proximity probe locatedadjacent a conductive target material to obtain a first digital voltagecorrelative to the first voltage at a first frequency; sampling anddigitizing a second voltage impressed only across the probe to obtain asecond digital voltage correlative to the second voltage at the firstfrequency, digitally convolving the first digital voltage and the seconddigital voltage into a first complex voltage number and a second complexvoltage number; calculating an electrical impedance of the proximityprobe at the first frequency by using the first complex voltage numberand the second complex voltage number; sampling and digitizing a thirdvoltage impressed across the serial connection of the resistance meansand the proximity probe located adjacent the conductive target materialto obtain a third digital voltage correlative to the third voltage at asecond frequency; sampling and digitizing a fourth voltage impressedonly across the probe to obtain a fourth digital voltage correlative tothe fourth voltage at the second frequency, digitally convolving thethird digital voltage and the fourth digital voltage into a thirdcomplex voltage number and a fourth complex voltage number; calculatinga complex electrical impedance of the proximity probe at the secondfrequency by using the third complex voltage number and the fourthcomplex voltage number; dividing a reactance of the calculated complexelectrical impedance of the sensing coil at the first frequency by thereactance of the calculated complex electrical impedance of the sensingcoil at the second different frequency for defining an inductive ratio;correlating the inductive ratio to a value representative to a gapbetween the proximity probe and the conductive target material.
 112. Amethod for measuring a gap between a proximity probe and a conductivetarget material, the steps including: providing a proximity probe havinga first end located adjacent a conductive target material and having asecond end coupled to a first end of an extension cable; measuring animpedance at a second end of the extension cable; compensating themeasured impedance by mathematically eliminating extension cableresiduals from the measured impedance for defining a proximity probeimpedance of the proximity probe; correlating the proximity probeimpedance with a value representative of a gap between the proximityprobe and the conductive target material.
 113. The method of claim 112wherein the step of compensating the measured impedance bymathematically eliminating the extension cable residuals includes thesteps of determining a first impedance of the extension cable with thefirst end opened for defining an open impedance, determining a secondimpedance of the extension cable with the first end shorted for defininga short impedance, and determining a third impedance of the extensioncable with the first end coupled to a load having a known value fordefining a load impedance.
 114. The method of claim 113 wherein the stepof compensating the measured impedance by mathematically eliminating theextension cable residuals from the measured impedance further includesthe step of determining the proximity probe impedance as a function ofthe measured impedance, the open impedance, the short impedance and theload impedance for defining the proximity probe impedance of theproximity probe.
 115. The method of claim 112 wherein the step ofcompensating the measured impedance by mathematically eliminating theextension cable residuals includes the step of using compensationcoefficients stored in a memory means for defining the proximity probeimpedance as a function of the compensation coefficients.
 116. Themethod of claim 115 wherein the step of using compensation coefficientsstored in a memory means includes the steps of determining a firstimpedance of the extension cable with the first end opened for definingan open impedance coefficient, determining a second impedance of theextension cable with the first end shorted for defining a shortimpedance coefficient, and determining a third impedance of theextension cable with the first end coupled to a load having a knownvalue for defining a load impedance coefficient wherein the openedimpedance coefficient, the short impedance coefficient and the loadimpedance coefficient are included in the compensation coefficientsstored in the memory means for defining the proximity probe impedance.117. A method for measuring a gap between a proximity probe and aconductive target material, the steps including: providing an extensioncable having two ends; determining a first impedance of the extensioncable with one of the two ends opened for defining an open impedance;determining a second impedance of the extension cable with one of thetwo ends shorted for defining a short impedance; providing a proximityprobe having an end located adjacent a conductive target material andhaving an opposite end coupled to one of the two ends of the extensioncable; measuring an impedance at the other end of the extension cable;determining an impedance of the proximity probe as a function of theshort impedance, the open impedance and the measured impedance fordefining a proximity probe impedance; correlating the proximity probeimpedance with a value representative of a gap between the proximityprobe and the conductive target material.
 118. The method of claim 117wherein the step of determining the impedance of the proximity probe asa function of the short impedance, the open impedance and the measuredimpedance for defining the proximity probe impedance includes the stepof calculating the impedance of the proximity probe from compensationcoefficients representative of the short impedance, the open impedanceand the measured impedance for defining the proximity probe impedance.119. A method for measuring a gap between a proximity probe and aconductive target material, the steps including: providing an extensioncable having two ends; determining a first impedance of the extensioncable with one of the two ends opened for defining a open impedance;determining a second impedance of the extension cable with one of thetwo ends shorted for defining a short impedance; determining a thirdimpedance of the extension cable with one of the two ends coupled to aload having a known value for defining a load impedance; providing aproximity probe having an end located adjacent a conductive targetmaterial and having an opposite end coupled to one of the two ends ofthe extension cable; measuring an impedance at the other end of theextension cable; determining an impedance of the proximity probe as afunction of the short impedance, the open impedance, the load impedanceand the measured impedance for defining the proximity probe impedance;correlating the proximity probe impedance with a value representative ofa gap between the proximity probe and the conductive target material.120. The method of claim 119 wherein the step of determining animpedance of the proximity probe as a function of the short impedance,the open impedance, the load impedance and the measured impedance fordefining the proximity probe impedance includes the step of calculatingthe impedance of the proximity probe from compensation coefficientsrepresentative of the short impedance, the open impedance, the loadimpedance and the measured impedance for defining the proximity probeimpedance.
 121. A method for measuring a gap between a proximity probeand a conductive target material, the steps including: providing anextension cable having two ends; determining a first load impedance ofthe extension cable with one of the two ends coupled to a first load;determining a second load impedance of the extension cable with one ofthe two ends coupled to a second load; the second load having animpedance that is less than the impedance of the first load; providing aproximity probe having an end located adjacent a conductive targetmaterial and having an opposite end coupled to one of the two ends ofthe extension cable; measuring an impedance at the other end of theextension cable; calculating a proximity probe impedance of theproximity probe as a function of the measured impedance, the first loadimpedance and the second load impedance for compensating for extensioncable residuals; correlating the proximity probe impedance with a valuerepresentative of a gap between the proximity probe and the conductivetarget material.
 122. The method of claim 121 further including the stepof determining a third load impedance of the extension cable with one ofthe two ends coupled to a third load, the third load having an impedancethat is less than the impedance of the first load and greater than theimpedance of the second load.
 123. The method of claim 122 wherein thestep of calculating the proximity probe impedance of the proximity probeas the function of the measured impedance, the first load impedance, andthe second load impedance for compensating for extension cable residualsincludes calculating the proximity probe impedance of the proximityprobe as a function of the measured impedance, the first load impedance,the second load impedance, and the third load impedance for compensatingfor extension cable residuals.
 124. A method for measuring acharacteristic of a conductive target material disposed adjacent aproximity probe, the steps including: providing a length of cable havinga first end and a second end; determining a first impedance of the cablewith the first end opened for defining a open impedance; determining asecond impedance of the cable with the first end shorted for defining ashort impedance; coupling the first end of the cable to a proximityprobe and having the second end of the cable coupled to a digital eddycurrent proximity system; measuring, at the second end of the cable, animpedance of the coupled cable and proximity probe; calculating theproximity probe impedance as a function of the measured impedance, theopen impedance, and the short impedance for compensating for cablelength residuals; correlating the proximity probe impedance with acharacteristic of a conductive target material disposed adjacent theproximity probe.
 125. The method of claim 124 further including the stepof determining a third impedance of the cable with the first end coupledto a load having a known value for defining a load impedance.
 126. Themethod of claim 125 wherein the step of calculating the proximity probeimpedance as a function of the measured impedance, the open impedance,and the short impedance for compensating for cable length residualsincludes calculating the proximity probe impedance as a function of themeasured impedance, the open impedance, the short impedance, and theload impedance for compensating for cable length residuals.